Immunology Unit EXAM 2 Flashcards

Question

What type of heavy chain does IgM have and list the specifics. Also it has an odd quirk due to the size of the Ig,- describe.

Answer 1

- IgM contain mu heavy chains; one variable and four constant domains. The mu heavy chain is glycosylated and contain a cystein-rich tail piece involved in the pentamer formation. - Has a cysteine- rich J (joining) peptide to aid in the pentamer formation.

Answer 2

- IgA1 (monomer) is found in circulation - IgA2 (dimer) is the predominant antibody in mucosal secretions - IgA2 similarly to IgM uses a J chain to stabilize the dimer - Alpha heavy chains

Answer 3

The submucosal plasma cell secretes IgA2 dimers which bind to the polyIg receptor on the luminal side of mucosal epitheilial cells. The bound complex gets endocytosed and the receptor is later cleaved while keeping the secretory component (SC) associated with IgA2. The SC stays bound after traversing the mucosal epithelial cell and is secreted. SC provides the IgA2 protection from proteolysis.

Answer 4

Is when the IgA complexes deposit in the mesangial region of the kidney --> activating complement and leading to kidney failure "glomerulonephritis" "IgA nephropathy". Incidence is 50%

Answer 5

IgD is relatively rare and has delta heavy chains. Found almost exclusively on various stages of B cell maturation. It has a monomeric immunoglobulin structure and is glycosylated.

Answer 6

Data suggests IgD functions as a co-stimulator in antigen presentation and acts in B cell development, and may also be something of an evolutionary relic.

Answer 7

- anti-IgD increased PLT count. -

Answer 8

high affinity Fc receptors on the surface of basophils and mast cells and to moderate affinity receptors on the surface of lymphocytes and monocytes.

Answer 9

- hypersensitivity, allergies, and asthma, due to histamine release caused by antigen binding to mast cell or basophil-associated IgE. - asthma and allergies. -

Answer 10

cleaves IgG in the hinge region between the first two heavy chain constant domains, yielding two antibody-binding (Fab) fragments and one fragment containing the constant region (Fc).

Answer 11

heavy chain beyond interchain disulfide bonds, yielding one fragment which can bind two antibody molecules ( F(ab')2 ) and cleaves the remainder of the heavy chain into small peptides

Answer 12

1, 2 respectively

Answer 13

- present in all healthy members of a species - represent separate genes 1. Light chain isotypes - Kappa or Lambda. Markers present in the constant domains of the light chains. 2. Heavy chain isotypes - markers present in the constant domains of heavy chains, distinguishing classes and subclasses Note: the "immunoglobulin isotype" refers to the phenotypic variations in the constant regions of the heavy and light chains.

Answer 14

- variations within a species involving different alleles at a given locus. Not all members of the same species have a particular allotype. Markers are found in heavy chain constant regions Note: allotype refers to the idea that each immunoglobin has unique sequences particular to the individual's genome that manifest in its constant region (normally).

Answer 15

variations associated with the antibody binding site, especially the hypervariable region within the variable domain specific for the individual antibody clone (private idiotype) or may be shared (public idiotype) Note: idiotype describes the distinctive sequence and region that makes any immunoglobing/TCR unique from others of the same type which is its variable region.

Answer 16

Specific for an epitope on an antigen.

Answer 17

Mice are introduced to an antigen and we ensure that antibodies are being produced. Then we take the spleen cells and fuse it with plasmacytoma cells. Fusion is rare so we start with tens of millions of cells to only get hundreds to thousands fused cells. To select for the hyrbidomas the plate has HAT medium (Hypoxanthine, Aminopterin and Thymidine; start up for de novo synthesis pathways). Lastly screen the remaining hybridomas for the relevant antibodies. Note: the spleen cells die in culture and plasmacytoma cells have mutant salvage pathway and the aminopterin blocks the de novo synthetic pathways.

Answer 18

A. Antigenicity = ability to bind an antibody | B. Immunogenicity = ability to stimulate an immune response (many small molecules are antigenic but not immunogenic)

Answer 19

Properties of antigens that contribute to immunogenicity – foreigness, molecular size, chemical complexity, degradability Other factors in immunogenicity – method of antigen administration, genetic constitution of immunized animal, amount of antigen, presence of adjuvant

Answer 20

A molecular surface on the antibody (paratope) interacts with a molecular surface on the antigen (epitope)

Answer 21

sum of all attractive and repulsive forces within one epitope-paratope association Affinity = monovalent binding (one epitope + one paratope) Affinity calculations: K = [AbAg] / [Ab][Ag]

Answer 22

composition and configuration. Change either and you loose affinity.

Answer 23

- Avidity = multivalent binding (increases stability of complex) - multimeric antibodies such as IgA2 and IgM can bind up to 4 and 10, respectively.

Answer 24

- Specificity: the relationship between the affinity of an antibody towards the protein to which it was raised and the affinity of that antibody towards other nonrelated proteins. A highly specific antibody will bind very tightly to one protein and not very tightly to most all other proteins. - Antiserum specificity: an antiserum is composed of many different antibodies, and the specificity of the antiserum towards any one antigen is determined by how many antibodies recognize that antigen with high affinity, as compared to any other antigens that are also recognized by the different antibodies in the antisera

Answer 25

an antibody raised against one protein binding to another protein. Antigens may share the same or similar epitope or the antibody has polyfunctional sites where the variable region can bind more than one epitope.

Answer 26

1. Chlamydia outer membrane protein contains an epitope similar to cardiac myosin heavy chain; antibodies raised against this epitope during chlaymidia infection can also bind cardiac muscle and lead to myocarditis 2. hepatitis B viral protein contains an epitope similar to myelin basic protein; antibodies raised against this epitope during viral infection can also bind myelin membranes and can lead to demyelinating diseases such as multiple sclerosis 3. a herpesvirus protein contains an epitope similar to an epitope on the nicotinic acetylcholine receptor; antibodies raised against this epitope during viral infection can also bind to nicotinic acetylcholine receptors at the neuromuscular junctions and cause neuromuscular diseases such as Myasthenia gravis

Answer 27

broad recognition of molecular patterns associated with pathogens, while the adaptive components (antibody and T cell responses) are highly antigen specific.

Answer 28

- Pattern recognition key: unique pathogen-associated molecular patterns (PAMPs) exhibited by microorganisms are recognized by pattern recognition receptors (PRRs) of the innate response. - the innate response can recognize danger associated molecular patterns (DAMPS) that represent sterile (non-infectious) evidence of cellular stress.

Answer 29

- antigen receptors (TCR and Ig) - the innate response. note: to maintain homeostasis the innate and adaptive immune system need to interact. Innate response also activates the adaptive response.

Answer 30

The non-induced innate mechanisms include physical and chemical barriers to infection that are always present. The induced innate mechanisms involve the activation of components through the interactions of PAMPs with PRRs.

Answer 31

Skin: Keratinized layers with tight junctions between cells. Intact skin is the most important barrier to infection -barrier compromise (e.g. burns) leads to severe systemic infection. 2. Mucous membranes - less efficient barriers than skin a. Intact membranes can be colonized b. Can be penetrated by some organisms c. Integrity of the membranes easily disrupted d. Major site of entry of infectious agents

Answer 32

Washing action of body fluids: tears, saliva, mucus, urine | 2. Surface motion: peristalsis, mucociliary escalator, swallowing

Answer 33

Enzymes - digestive tract, lysozomal, tissue secretions (e.g. lysozyme) 2. Competition and interference - glycoproteins, lactoferrin, transferrin, interferon, normal microflora 3. pH - lactic acid, fatty acids lower pH 4. Defensins: cysteine-rich peptides present at epithelial surfaces and in neutrophil granules. Broad-spectrum antibiotics that kill wide variety of microorganisms.

Answer 34

Examples: 1. CRP (C-Reactive protein): binds the C protein of Pneumococci, facilitating uptake by phagocytes 2. Complement components: certain complement components increase during infection; a number of micro-organisms activate the classical or alternate complement pathway 3. α-2 macroglobulin and other anti-proteinases increased 4. IL-6 regulation of fibrinogen: increased fibrinogen leads to increased erythrocyte sedimentation rate. "Black bile" - thought to be dangerous and was ancient rationale for bleeding patients.

Answer 35

monocytes, macrophages, natural killer cells, dendritic cells, eosinophils, neutrophils. Some of these play important part in the adaptive responses as well. A new category of innate cells called “innate lymphoid cells”, of which NK cells is the prototype

Answer 36

PRRs include C-type lectin receptors of which the mannose receptor is the prototype. These C-type lectin receptors recognize repeating carbohydrates on pathogens. The Toll-like receptors (TLR 1-10) are important pattern recognition receptors. Some of the TLR are expressed on the cell surface, while others (TLR 3,7,8,9) are expressed in the endosomes. These highly conserved receptors are expressed selectively on different cell types and recognize structures/components of pathogens

Answer 37

TLR-4 is the receptor for bacterial LPS while TLR-9 is the receptor for bacterial and viral DNA sequences (CpG) that are non-methylated. TLR2 recognizes a variety of bacterial components and TLR3 recognizes double-stranded RNA, a replicative intermediate for many viruses.

Answer 38

DNA and RNA sensors.

Answer 39

- endogenous factors associated with molecular stress such as ATP released from dying (necrotic cells) and uric acid crystals. - inflammasomes

Answer 40

a. Neutrophils - differentiate in bone marrow; short lived. Function early in innate response. b. Macrophages i. Bone marrow precursors, long-lived ii. Differentiate from blood monocytes to fixed or mobile cells iii. Heterogeneous in range and scope of function iv. Function later in immune response and are important bridge to adaptive immunity -myeloid lineage.

Answer 41

Chemotaxis - movement toward a concentration gradient of a chemical signal b. The signals involved are of diverse nature and are derived from a number of sources, e.g.: bacterial factors, tissue injury factors, cytokines, chemokines. c. Chemical signals are recognized and responded to by means of receptors at the cell surface. d. Chemotactic signals induce biochemical events leading to the polymerization of microtubules and the contraction and relaxation of microfilaments which in turn allow for the formation of pseudopodia and locomotion.

Answer 42

Phagocytes adhere to the endothelium, first loosely, then more tightly and dissolve the basement membrane, allowing them to pass from the vessel toward the site of inflammation (diapedesis or extravasation). Adherence to the epithelium mediated through adhesion molecules that are upregulated on epithelium in response to infection/cytokines

Answer 43

At the point of adherence, microtubules are formed and micro-filaments begin contraction-relaxation cycle b. Pseudopods draw agent into intracellular, membrane-bound vesicle called a phagosome c. Not all phagocyte/micro-organism binding leads to phagocytosis

Answer 44

primary lysosomal granules of the cell migrate toward and fuse with phagosome, discharging their contents into the vesicle, now called a phagolysosome.

Answer 45

Increase in oxygen consumption b. Activation of the hexose-monophosphate shunt c. Activation of the nicotinamide adenine dinucleotide system (NADP) d. Increase in glycolysis

Answer 46

These include a wide array of hydrolytic enzymes which can act on a wide range of substrates, thus inactivating and digesting them down to component residues. Most of these enzymes act at acid pH (3.5 -4.5) and this pH is generated in the phagolysosome by the formation of lactic acid during the “respiratory burst”.

Answer 47

phagocyte oxidase system. Phagocyte oxidase is upregulated by IFN-gamma, signaling through TLRs, which leads to the generation of toxic free radicals.

Answer 48

(a) Acid: lactic acid via glycolytic activity (b) Lysozyme - an enzyme which hydrolyzes beta 1,4 linkages between carbohydrate residues in the cell walls of many bacteria. Active against gram-positive organisms. (c) Lactoferrin - competes with microbial cells for iron which is essential to many bacteria. (d) Cationic proteins - "Defensins" also active here. These are a diverse family of basic proteins such a phagocytin and leukin which have antimicrobial activity. Some of these act as detergent-like agents at the cell membranes by forming ion-permeable channels. Most active at pH 7.0 before acidification. Active against diverse organisms

Answer 49

a. Synthesis and secretion of nitrogen oxides carried out by activated macrophages induced by LPS and IFN-γ. Unstable free radicals involved in prevention of platelet aggregation, and defense against parasites, tumor cells and intracellular bacteria. b. Produced through the action of inducible NO synthetase. Converts L-arginine to citrulline + nitric oxide. c. Major cytotoxic effector against tumor cells, parasitic fungi, protozoa, helminths, mycobacteria bacteria. May also be mediator of autoimmunity through destruction of healthy tissue. d. Mechanism of action: inhibition of Krebs cycle, electron transport, DNA synthesis. e. Evasion: streptomyces make N-iminoethyl -L- ornithine which is an irreversible inhibitor of NO synthase in macrophages.

Answer 50

Neutrophil activation can lead to absess, which is liquified tissue, neutrophils, cell debris - "pus"

Answer 51

Unlike T and B cells, they don’t

Answer 52

Cells of the lymphoid lineage capable of recognizing and killing certain virus infected cells, tumor cells, some normal cells and certain bacteria, fungi, parasites. Note: Belong to the innate immune system in that they are relatively non-specific and don't exhibit classical memory, but recent study show a modified type of memory.

Answer 53

NK in addition to killing, produce IFN-γ, GM-CSF, TNF as well as other cytokines. IFN-γ, for example activates macrophage killing.

Answer 54

NK cells have receptors for the Fc region of IgG antibody (FcR) allowing them to bind Ab and mediate antibody - dependent cellular cytotoxicity (ADCC). This FCR receptor is called “CD16”.

Answer 55

NK cells recognize lack of MHC Class I molecules. Target cells that have MHC I engage killer inhibitory receptors (KIR) and negatively signal NK ---> No kill. Killing results from engagement of killer activating receptors (KAR) and failure to negatively signal through KIR.

Answer 56

perforins, proteases; very similar to lysis by CTL (cytotoxic T cell)

Answer 57

Released from several cell types in response to virus infection, DS RNA, endotoxin,mitogenic and antigenic stimuli. Can augment immune response (NK, B, macrophage); lead to an antiviral effect. 2. Type I IFN: IFN-α, β ---> signal through the IFNAR. Made by leukocytes, dendritic cells, monocytes, fibroblasts. IFN-γ ----> signals through IFNGR. Made primarily by CD4+ T cells, NK cells. Type III IFN: IFN-λ; active at mucosal surfaces, in pDC 3. IFNs act as links between innate, adaptive response. IFN production by a specialized dendritic cell subtype (the plasmacytoid dendritic cell, PDC) primes for development of Th1 response.

Answer 58

Produced mostly by activated macrophages but also NK cells. Roles in endotoxic shock, inflammation, immunoregulation, antiviral responses, fever.

Answer 59

Produced by macrophages, T cells, endothelial cells causes increases in bone marrow production of neutrophils and stimulates production of fibrinogen, CRP (acute phase proteins), induces fever.

Answer 60

Produced upon activation of the inflammasome. Along with TNF-α and IL-6, it is an “endogenous” pyogen or fever-inducing agent through the activation of prostaglandin E2. Induces acute phase proteins, mobilizes neutrophils. [In fact, TNF, IL-1 and IL-6 have a lot of overlapping functions, demonstrating the concept of cytokine pleiotropy and redundancy.

Answer 61

Produced by monocytes and other antigen presenting cells, particularly the type I dendritic cell populations. Stimulates cytotoxic cell activity and proliferation of T cells and NK cells. One of the major links between innate and adaptive immunity, leading to development of TH1 responses and cell mediated immunity.

Answer 62

- The Complement system is an important collection of serum proteins that work in concert with other elements of the immune system to initiate effector mechanisms that result in the destruction of extracellular infectious organisms. - the complement system can also act in the absence of specific antibodies based on binding of complement proteins to certain surface molecular patterns expressed by pathogens.

Answer 63

in an inactive form, in the absence of infection. In the presence of a pathogen, these proteins are “activated” in a sequential manner to non-specifically complement the action of antibodies in both opsonization and lysis of red blood cells and bacteria.

Answer 64

In the process of being activated, many of the complement proteins release low molecular weight peptides (split products) which themselves have powerful activating effects on inflammatory cells.

Answer 65

Hepatic parenchyma cells. - C1 ; in the epithelium of the gastrointestinal and urogenital tracts. - monocytes or macrophages

Answer 66

- classical, alternative, lectin 1. Opsonization and phagocytosis 2. Stimulation of inflammatory reactions 3. Complement-mediated cytolysis (osmotic lysis) 4. Clearance of immune complexes

Answer 67

1. Activation of complement involves the sequential proteolysis of proteins to generate enzymes with proteolytic activity. 2. The products of complement activation become covalently attached to microbial cell surfaces or to antibodies bound to microbes and to other antigens. 3. Complement activation is inhibited by regulatory proteins that are present on normal host cells and absent from microbes.

Answer 68

A. The first proteins discovered belong to the classical pathway and are designated by the letter C followed by a number. B. Products of cleavage reactions are designated by adding a lowercase letter as a suffix. Cleavage of C3 produces a small protein fragment called C3a and a larger fragment C3b. In general, the smaller cleavage fragment is designated by the suffix a, and the larger fragment by the suffix b. C. Some exceptions, C2, larger fragment is C2a. C1q, C1r and C1s are not cleavage products of C1 but are distinct proteins that comprise C1. D. Components of the Alternative Pathway are called “factors” and are distinguished by capital letters (e.g., Factor B; Factor D). Cleavage fragments are designated by adding a lowercase letter as a suffix.

Answer 69

Is an oligomeric lectin with collagen and fibrinogen subunits. Lectin are carbohydrate binding proteins.

Answer 70

A. Lectin pathway – initiated by soluble carbohydrate-binding proteins – mannose binding lectin and the ficolins that bind to carbohydrates on microbial surfaces. B. Classical pathway – initiated when the complement component C1 either recognizes a microbial surface directly or binds to antibodies bound to a pathogen. C. Alternative pathway – initiated by spontaneous hydrolysis and activation of complement component C3 which can bind directly to microbial surfaces.

Answer 71

most important step in complement activation – the generation of C3 convertase.

Answer 72

One regulatory safeguard is that the key activated complement components are rapidly inactivated unless they bind to the pathogen surface on which their activation was initiated.

Answer 73

1 lectin and 3 ficollins

Answer 74

Mannose binding lectin complexes with Mannose assoc. serine protease 1/2. MBL binding to a pathogen surface induces a conformational change in MASP-2 which can then cleave complement components C4 and C2. Cleavage of C4 releases C4a and C4b, whereas C2 is cleaved producing C2a, an active serine protease which binds C4b, forming C4b2a. C4b2a is the C3 convertase of the lectin pathway.

Answer 75

have more respiratory infections by common extracellular bacteria during early childhood.

Answer 76

Mannose-binding lectin (MBL) is an oligomeric protein built up from a monomer that contains an amino-terminal collagen-like domain and a carboxy-terminal C-type lectin domain. Collectins is another name for these proteins. MBL in the blood is composed of 2-6 trimers that bind with high avidity to repetitive carbohydrate structures on a variety of microbial surfaces, such as Gram-positive and Gram-negative bacteria, mycobacteria, yeasts and some viruses and parasites. MBL is present in low concentrations in the plasma but in the presence of an infection its production is increased.

Answer 77

They contain a fibrinogen-like domain attached to the collagen-like stalk. Ficollins have specificity for oligosaccharides containing acetylated sugars.

Answer 78

This pathway uses a pathogen sensor known as the C1 complex. The C1 complex is composed of the C1q subunit, which acts as the pathogen sensor and two serine proteases C1r and C1s. C1r and C1s are closely related to MASP-2. C1q interacts with a ligand, which leads to activation of C1r that cleaves C1s to generate an active serine protease. Activated C1s cleaves C4 to produce C4b which binds to the pathogen surface. C4b and cleaves C2 to produce C2a. C4b binds to C2a that produces the active C3 convertase C4bC2a.

Answer 79

The components of the Classical C pathway are numbered C1-C9, and were so named in order of their discovery rather than their activation sequence (C1, 4, 2, 3, 5, 6, 7, 8, 9) which became apparent later.

Answer 80

1. Can bind directly to surface components on some bacteria and to polyanionic structures such as lipoteichoic acid. 2. Can bind to C-reactive protein, an acute phase protein in human plasma that bind phosphocholine residues in bacteria surface molecules. 3. Can bind to the constant regions of antibodies (Fc region) that have bound pathogen via their antigen-binding sites. Human — IgGl, IgG3 and IgM Mice — IgG2a and IgM

Answer 81

Each Ig Fc region has a single Clq-binding site, and each Clq molecule must bind to two Ig heavy chains to be activated. This requirement explains why antibodies bound to antigens and not free circulating antibodies can initiate classical pathway activation.

Answer 82

Key features of this pathway are 1. its ability to be spontaneously activated 2. its unique C3 convertase, the alternative pathway C3 convertase. The cleavage fragment of Factor B is Bb which binds to C3b to form C3bBb. 3. Once C3b has been formed by any pathway, the alternative pathway can acts as an amplification loop to increase C3b production.

Answer 83

1. by the action of either the lectin or classical pathway. C3b generated by either pathway and covalently linked to a microbial surface can bind factor B. This binding enables factor D to cleave it into Ba or Bb. Bb remains associate with C3b, forming the C3bBb convertase. 2. The second way involves the spontaneous hydrolysis (“tickover”) of the thioester bond in C3 to form C3(H2O). C3(H2O) binds factor B, which is cleaved by factor D producing fluid-phase C3 convertase, C3(H2O)Bb.

Answer 84

Alternative pathway C3 convertases are very short-lived. They are stabilized by the plasma protein properdin (factor P), which binds to C3 and C3(H2O). Properdin is made by neutrophils. Properdin-deficient patients are very susceptible to infections with Neisseria meningitides, the main agent of bacterial meningitis.

Answer 85

A. C3b binds to C4b2a and C3bBb forming the active C5 convertases C4b2a3b and C3b2Bb. B. C5 binds to the C3b component of the C5 converting enzyme. C. C5 is cleaved by C2a or Bb to form C5a and C5b. D. Main effect of complement activation is to deposit large quantities of C3 on the surface of the pathogen that signals the destruction of the pathogen.

Answer 86

Most important action of complement is to facilitate the uptake and destruction of pathogens by phagocytic cells. This occurs by the specific recognition of bound complement components by complement receptors (CRs).

Answer 87

CRl, or CD35 functions mainly to promote phagocytosis of C3b- and C4b-coated particles and clearance of immune complexes from the circulation. Expressed on many types of immune cells including macrophages and neutrophils. C5a activates macrophages to ingest bacteria bound to their CR1 receptors. Note: C5a receptor transducer their signals via intracellular guanine-nucleotide-binding proteins called G proteins.

Answer 88

They bind to inactivated forms of C3b that remain attached to the pathogen surface. Binding of iC3b to CR3 stimulates phagocytosis.

Answer 89

C3a, like C5a signals through G-protein-coupled receptors. In large amounts or if injected systemically they produce a shock-like syndrome similar to a systemic allergic reaction involving IgE antibodies. This reaction is termed anaphylactic shock and the complement components C3a and C5a are referred to as anaphylotoxins. During this reaction, there is smooth muscle contraction, increased vascular permeability, upregulation of adhesion molecules and release of inflammatory molecules such as histamine and TNF-α.

Answer 90

- One of the most important effects of complement activation is the assembly of the terminal complement components to form a membrane-attack complex. - End result is a pore in the lipid bilayer membrane that destroys membrane integrity.

Answer 91

C5 is cleaved by a C5 convertase to release C5b. One molecule of C5b binds to one molecule of C6 and the C5b6 complex binds to C7. This results in a conformational change allowing C7 to insert itself into the lipid bilayer. C8 binds to the C5b6 complex and inserts into the lipid bilayer. C8 molecule binding to the complex induces the polymerization of 10-16 molecules of C9. The 10-16 molecules form a pore in the membrane and constitute the membrane-attack complex.

Answer 92

There's an association with susceptibility only to Neisseria species, the bacteria that causes the sexually transmitted disease gonorrhea and a common form of bacterial meningitis.

Answer 93

that complement activation occurs only on the surface of a pathogen or on damaged host cells and not on normal host cells and tissues and to limit the duration of complement activation even on microbial cells and antigen-antibody complexes.

Answer 94

Complement activation by any pathway and may amplified via the alternative pathway. Extent of amplification is dependent on the stability of C3 convertase C3bBb.

Answer 95

Positive and negative regulatory proteins control the stability of C3 convertase C3bBb. Negative regulatory proteins protect host cells from complement damage. Complement regulatory proteins interact with C3b and either prevent the convertase from forming or promote its rapid dissociation.

Answer 96

Properdin acts as a positive regulatory protein and stabilizes C3bBb.

Answer 97

Decay-accelerating factor (DAF or CD55) competes with factor B for binding to C3b and can displace Bb from a convertase that has already formed.

Answer 98

C3b can be cleaved to an inactive form iC3b by a plasma protease, factor I and membrane cofactor of proteolysis (MCP or CD46).

Answer 99

Cell-surface complement receptor type 1 (CR1)

Answer 100

Factor H binds C3b and competes with factor B to displace Bb from the convertase.

Answer 101

- C1 inhibitor (C1INH) is a plasma serine protease inhibitor that binds the active enzymes C1r:C1s and causes dissociation from C1q. Limits the time during which active C1s is able to cleave C4 and C2. - C1INH deficiency disease – hereditary angioedema (HAE) causes extensive swelling in the larynx and can lead to suffocation.

Answer 102

Cell-associated C3b (and C4b) are proteolytically degraded by a plasma serine protease called Factor I, which is active only in the presence of regulatory proteins, such as CR1 and Membrane Cofactor Protein (MCP). Factor I deficiency – uncontrolled complement activation resulting in repeated bacterial infections, especially pyogenic bacteria.

Answer 103

Formation of the MAC is inhibited by a membrane protein called protectin (CD59), which works by inhibiting addition of C9 molecules; and by plasma proteins, such as S-protein, that binds to soluble C5b, 6, 7 complexes and hereby prevents their insertion into the cell membrane.

Answer 104

promote phagocytosis of microbes on which complement is activated, stimulate inflammation, and induce the lysis of these microbes.

Answer 105

Opsonization and Phagocytosis: Microbes on which complement is activated by the alternative or classical pathway become coated with C3b, iC3b, or C4b and are phagocytosed by the binding of these proteins to specific receptors on macrophages and neutrophils.

Answer 106

Stimulation of Inflammatory Responses: The proteolytic complement fragments C5a and C3a, and C4a are known as anaphylatoxins because they induce acute inflammation (C5a > C3a > C4a). Myeloid cells are attracted to areas where these C split products are released, and increased adhesion and activation of neutrophils, and mast cell degranulation also occurs. Such reactions form the basis for the pathology that is observed in some of the hypersensitivity states associated with many diseases.

Answer 107

Complement-Mediated Cytolysis: Complement-mediated lysis of foreign organisms is mediated by the MAC.

Answer 108

Solubilization of immune complexes: During an antibody response, antigen-antibody (i.e., immune) complexes are formed. If these complexes accumulate in the blood, they may deposit in blood vessel walls, leading to inflammatory reactions that can damage surrounding tissue. Complement activation on Antibody molecules can promote dissolution of immune complexes to prevent such damage from occurring.

Answer 109

Detection or identification of antigens and specific antibodies are used for DIAGNOSIS OF INFECTION and quantitative assays are used to measure the SEVERITY OF INFECTION or HOST IMMUNE RESPONSE. note: A large number of these tests are based on antigen-antibody interactions, while others measure cellular competence.

Answer 110

Cell flow cytometry.

Answer 111

percentages and absolute.

Answer 112

Flow cytometry enumerates different cell types. It measures characteristics of cells as they flow in a single file past a laser beam. Immunologists use flow cytometry to determine the types of markers and receptors on the surface of a cell. A fluorescently labeled antibody specific for a surface marker or intracellular protein is used to identify the cell.

Answer 113

Unlabeled cells scatter the light as they pass through the laser in the nozzle of the flow cytometer. Information on the size and complexity (granularity) of a cell is obtained by the way the light is scattered. ``` side = complexity/granularity forward = size ```

Answer 114

- 2 labels ==> scattergram | - 6 to 8 different dyes

Answer 115

flow results on patients via flow cytometry

Answer 116

quantitative analysis of lymphocyte subsets, using surface markers CD antigens • immunophenotyping of leukemia’s and lymphomas • defines early stages in the development and lineage of T AND B Lymphocytes • monitoring of HIV infected and immunocompromised patients • DNA analysis (ploidy) • Intracellular flow • Fluorescence-activated cell sorting (FACS) for isolating different cell populations

Answer 117

immune complex

Answer 118

Almost equivalent concentrations of antigen and antibody

Answer 119

- particulate antibody coated on surface of latex beads or cells. - uses RBC as an endpoint of detection

Answer 120

hemagglutination

Answer 121

entails clumping of beads/cells more sensitive than precipitin reactions Applications: blood typing, coombs direct antiglobulin test (for fetal Rh reactions) measurement of Rheumatoid factor, C reactive protein, antibodies to Treponema pallidum (reagin)

Answer 122

Used currently in immunology Much more sensitive – detect pg. levels Uses mostly monoclonal antibodies Can be used to detect antigen or antibody or cells that make these molecules Applications: measure antibodies to HIV & Hepatitis and other viruses

Answer 123

Label is an enzyme e.g. peroxidase or alkaline phosphatase which reacts with a substrate to generate a signal – signal is amplified – increases sensitivity Immunoassay for antibody or antigen Bind antigen to plate Add first antibody (labeled – direct) Add secondary antibody (with label - indirect) Different labels – radioisotopes (RadioImmuno Assay – RIA) ELISPOT – Elisa around cells – forms spot ; Can measure antibody secreting cell but more widely used now for the single cell detection of cytokine secretion.

Answer 124

Electrophoretic separation of antigen (protein) by molecular weight Transfer to a nitrocellulose membrane Antibodies to specific proteins bind & are detected colorimetrically Applications: antibodies to HIV

Answer 125

Measure delayed type hypersensitivity responses Antigens (recall) against which people are exposed and develop memory cells Lymphocyte proliferation in response to specific antigens Cytokine production by CD4+ T cells -Applications: Quantiferon Gold test for TB; measurement of IFN-γ due to specific stimulation of T cells sensitized to TB Lymphocyte proliferation for (immunosuppresed) transplant recipients

Answer 126

Separation of serum proteins based on charge Albumin most negatively charged, followed by α, β and γ globulins Used to detect Ig spikes – myelomas IgG, IgA myeloma proteins IgM Waldenstrom’s macroglobulinaemia Applications: Monoclonal gammopathies Immunoelectrophoresis or Immunofixation uses antibodies to identify the type of immunoglobulin and the light chain utilized

Answer 127

The major histocompatibility complex (MHC) is a large genetic region that encodes class I and II molecules and other proteins. The MHC genes are extremely polymorphic - many alleles existing for each gene and polygenic - many different MHC genes exist (cluster of genes).

Answer 128

physiological role is to bind peptide antigens and present them for recognition by antigen-specific T lymphocytes. Peptide antigens associated with MHC class I molecules are recognized by CD8+ cytotoxic T cells, whereas class II-associated peptide antigens are recognized by CD4+ helper T cells.

Answer 129

The structure of class I and class II molecules is similar. Class I molecules are expressed on all nucleated cells whereas class II molecules are expressed mainly on B cells, macrophages and dendritic cells.

Answer 130

The MHC was originally discovered as a locus containing highly polymorphic genes that determined the outcome of tissue transplants exchanged between individuals.

Answer 131

antigens on the white blood cells of the blood or organ donor.

Answer 132

- Sera that react against the cells of allogeneic individuals are called alloantisera and they contain alloantibodies, whose targets are called alloantigens. - human leukocyte antigens or HLA

Answer 133

Human MHC is the HLA gene region, while the mouse MHC is the H-2 region

Answer 134

Class I – HLA-A, HLA-B and HLA-C molecules. | Class II – HLA-DR, HLA-DQ and HLA-DP molecules.

Answer 135

Structurally distinct and homologous.

Answer 136

Haplotype is a set of DNA variation of polymorphisms that are inherited together and found on the same chromosome. The set of MHC alleles present on each chromosome is inherited as a unit. Trick: MHC molecules are expressed codominantly in each individual.

Answer 137

MHC genes have about 8-100 alleles identified at each locus.

Answer 138

Linkage disequilibrium is association of certain HLA alleles at different loci more frequently than would be predicted by random assortment.

Answer 139

- an extracellular peptide-binding cleft, an immunoglobulin-like region, a transmembrane region and a cytoplasmic region. - The polymorphic amino acid residues of MHC molecules are located in and adjacent to the peptide-binding cleft

Answer 140

The immunoglobulin-like domains of MHC molecules contain binding sites for the T cell molecules CD4 and CD8.

Answer 141

1. Consist of two noncovalently linked chains: an MHC-encoded alpha-chain (44-47 kD) and a non-MHC encoded beta-2 microglobulin (12 kD) . 2. Fully assembled molecule is a heterotrimer consisting of an alpha- chain, 2-microglobulin and a bound antigenic peptide. 3. The peptide-binding cleft resides in the alpha1 and alpha2 domains 4. The physiologic role of class I determinants is the presentation of antigenic peptides to CD8+ T cells. 5. Each normal (heterozygous) individual expresses six different class I molecules on every cell, derived from the two alleles of HLA-A, HLA-B and HLA-C genes that are inherited from the parents.

Answer 142

1. Consist of two noncovalently associated chains: a alpha-chain (32-34 kD) and a beta-chain (29-32 kD), both encoded by MHC genes. 2. The peptide-binding cleft resides in the alpha1 and beta1 domains. 3. Fully assembled molecule is a heterotrimer consisting of an alpha chain, a beta chain and a bound antigenic peptide. 4. The physiologic role of class II determinants is the presentation of antigenic peptides to CD4+ T cells. 5. Six class II MHC alleles are inherited, three (HLA-DR, DQ, DP) from each parent.

Answer 143

1. MHC molecules show a broad specificity for peptide binding, and the fine specificity of antigen recognition resides largely in the T cell antigen receptors. 2. Each class I or II MHC molecule has a single peptide-binding cleft that can accommodate many different peptides.

Answer 144

3. Class I molecules can accommodate peptides of 8-10 residues (Table 8-2 Kuby J, Immunology, 6th Edition). 4. Class II molecules can accommodate peptides of 13-18 residues (Table 8-2 Kuby J, Immunology, 6th Edition). 5. Peptides that bind to MHC molecules share structural features. 6. MHC molecules do not discriminate between foreign peptides and peptides derived from self antigens.

Answer 145

- Binding of peptides to MHC molecules is a noncovalent interaction mediated by residues both in the peptides and in the clefts of the MHC molecules (Fig. 8-7, Kuby J, Immunology, 6th Edition). - Anchor residues are the amino acid residues of a peptide whose side chains fit into pockets in the peptide-binding cleft of an MHC molecule

Answer 146

1. HLA region is located on the short arm of chromosome 6 and beta2-microglobulin is encoded by a gene on chromosome 15. 2. Many proteins involved in the processing and presentation of antigens are encoded by genes mapping within the MHC.

Answer 147

Transporter associated with antigen processing (TAP) is a heterodimer that transports peptides from the cytosol into the endoplasmic reticulum, where the peptides associate with the class I molecules (Fig. 6.15, Janeway C, Immunobiology, 8th Edition. 4. Other genes encode subunits of a cytosolic protease complex, called the proteasome that degrades cytosolic proteins into peptides that are presented by class I molecules. 5. HLA-DM is a nonpolymorphic heterodimeric class II molecule that is involved in peptide binding to class II molecules.

Answer 148

1. Class I molecules are constitutively expressed on the surface of all nucleated cells and platelets. 2. Class II molecules are constitutively expressed on dendritic cells, B cells, macrophages, activated T cells and a few other cell types. 3. Expression of MHC molecules is increased by cytokines produced during both innate and adaptive immune responses. 4. The rate of transcription is the major determinant of the level of MHC molecule synthesis and expression on the cell surface.

Answer 149

MHC molecules are the major determinants responsible for graft rejection. The high frequency of T cells reactive with allogeneic MHC molecules is the reason why allograft rejection is a strong response in vivo.

Answer 150

yes. The T cell receptor on a T cell that was selected to recognize self-MHC and foreign peptide may recognize an allogeneic MHC plus peptide.

Answer 151

1. Superantigens are a distinct class of antigens produced by many different pathogens (bacteria, viruses, mycoplasmas) that stimulate a primary T cell response similar in magnitude to a response to allogeneic MHC molecules. 2. Superantigens stimulate large numbers of T cells by binding simultaneously to MHC Class II molecules and certain Vβ domains of T cell receptors. 3. Mode of stimulation does not prime an adaptive immune response but causes production of cytokines resulting in systemic toxicity and suppression of the adaptive immune response. The responses they provoke are helpful to the pathogen and not the host.

Answer 152

tissue typing- is the technique used to determine the MHC alleles expressed on an individual’s cells. Serology based typing is performed by adding the cells to be tested (usually lymphocytes) to antisera of defined HLA specificity. Addition of complement kills the cells and this can be visualized by staining with a dye that is taken up by dead cells. Cell death indicates that the test cell carried the allele in question. - In molecular based typing genomic DNA is PCR amplified with specific primers to all the known alleles at each HLA locus (-A, -B, -C, -DRB, -DQB).

Answer 153

tissue typing also called lymphocytotoxicity testing can be used to detect anti-HLA antibodies in serum by testing the serum against a panel of lymphocytes with known HLA types.

Answer 154

is an in vitro assay for T cell recognition of allogeneic MHC molecules and is based on the fact that T lymphocytes are stimulated to grow in the presence of cells expressing allogeneic CLASS II MOLECULES. The test lymphocytes or responders are co-cultured with lymphocytes (stimulators) homozygous for HLA-DR molecules. The stimulators have been irradiated to prevent them from dividing in response to the responders. Responders lacking the HLA-DR allele of the stimulators recognize it as foreign and proliferate. Proliferation is measured by the uptake of 3H-Thymidine into DNA. Low or background amounts of radiolabel uptake indicate similarity between the responding and stimulating cells for HLA-DR molecules.

Answer 155

1. Paternity testing – HLA typing is very useful in cases of disputed paternity. 2. Disease associations – there appears to be an association between HLA type and the occurrence of certain diseases. Disease associations are expressed as the relative risk for individuals carrying a certain allele to develop the disease. 3. Organ transplantation – the MHC molecules are responsible for almost all strong rejection episodes. HLA matching, especially of HLA-B and HLA-DR molecules between the recipient and donor have an effect on graft survival

Answer 156

T cells can only respond to antigen that is associated with self-MHC

Answer 157

1. material taken-up from outside of the specialized antigen presenting cells is typically processed through an endosomal pathway and leads to expression of antigen-derived peptides expressed with MHC Class II 2. Antigens produced inside of the cell (e.g. viral antigens from viruses that have infected the cell) are processed through the proteosomes and are expressed with MHC Class I.

Answer 158

1. T cells recognize antigen bound to MHC molecules but cannot recognize non-MHC bound or soluble antigen whereas B cells and Ab can recognize free or soluble antigen in addition to cell-associated antigens. 2. T cells predominantly recognize protein antigens whereas B cells can recognize proteins, nucleic acids, polysaccharides, lipids and small molecules. 3. B cells can recognize either conformational or linear determinants whereas T cells recognize only linear determinants. T cells recognize “processed” antigen.

Answer 159

APC must express MHC molecules that the T cell recognizes as self in order for the T cell to recognize and respond to a foreign protein antigen presented by that APC. A. CD8+ T cells "see" antigen complexed with self MHC Class I molecules – “MHC Class I restricted” B. CD4+ T cells "see" antigen complexed with self MHC Class II molecules – “MHC Class II restricted” Note: "Self" MHC are those MHC molecules that the T cells encountered during development in the thymus.

Answer 160

- dendritic cells, B cells, macrophages - other cell types that upregulate MHC Class II (cells induced by IFN-gamma) Note: Langerhans Cells (type of DC) and some endothelial cells are APCs.

Answer 161

found as immature cells in the peripheral blood and as immature or mature cells in secondary lymphoid tissue. These cells are heterogeneous, with different subtypes having different phenotypes and functions - one subset of DC are Langerhans cells which are immature DC located in the skin. They capture antigens/pathogens in the skin and bring them to the lymph nodes where they mature to APC. - Other DCs are found at other portals of entry of pathogens (mucosal sites).

Answer 162

Mature DENDRITIC CELLS are most efficient of APCs for initiating immune response in naive T cells because of high levels of MHC class II and co-stimulatory molecules.

Answer 163

APC that phagocytose large particles including bacteria, parasites; upregulate class II expression in response to IFN-γ to become more efficient APC.

Answer 164

Use Ig on surface to bind specific Ag and internalize it. - Particularly important for Ab production in secondary immune response when Ag concentration is low but specific B cells are expanded. - B cells are not efficient in the primary immune response because antigen-specific B cells are present at very low frequency and have low expression of co-stimulatory molecules.

Answer 165

The APC not only allow binding of these cells to specific T cells but also provide costimulatory activity by membrane bound or secreted products of the APC (also called "accessory cells".)

Answer 166

"target cells" are then recognized and lysed by cytotoxic T cells. Since virtually all nucleated cells in the body express MHC Class I, they have the potential to present antigen to CD8+ T cells and become targets for lysis.

Answer 167

depends on whether the antigen is to be expressed in the context of MHC Class I or Class II molecules.

Answer 168

Class II MHC molecules present "exogenous antigens" (or antigens that are being taken up from the outside) while Class I MHC molecules present "endogenous antigens". Endogenous antigens are those proteins being synthesized within the cell, (for example, viral or tumor cell antigens). However, MHC molecules do not distinguish between self/non-self, so self-antigens are also constantly being presented as well.

Answer 169

- Antigens form physical complexes with MHC molecules. The APCs convert native proteins to MHC associated peptide fragments by a process termed antigen processing. - The interaction of MHC molecules with the processed peptides is non-covalent and depends on the physical properties of both the peptide and the "peptide groove" in the MHC molecule. - within a given antigen that has multiple epitopes, there are certain epitopes that characteristically bind with the highest avidity to MHC; these are immunodominant epitopes.

Answer 170

One MHC molecule can potentially bind many different peptides (but not simultaneously); however, not all peptides bind to a given MHC molecule

Answer 171

1. Exogenous antigen is internalized by phagocytosis or endocytosis. This can be facilitated by opsonization (for macrophages) or by specific interaction of antigen with cell surface immunoglobulin on B cells. 2. Antigen processing is a time and energy dependent process. 3. Processing takes place in acidic intracellular compartments 4. Cellular proteases (e.g. cathepsins) , many of which function best at acid pH, are required for processing of many protein antigens.

Answer 172

The processed antigens are usually proteolytically cleaved fragments which associate with MHC Class II molecules in membrane bound vesicles (endosomes) within the APC. - Either newly synthesized or recycling MHC Class II molecules associate with the processed antigen and are transported to the cell surface.

Answer 173

During biosynthesis of Class II molecules, an invariant or Ii chain associates transiently with the Class II α/β dimer and targets the α/β dimer to the endocytic pathway. Further, this prevents binding of peptides to the dimers in the ER; therefore, peptides from newly synthesized proteins do not usually get access to new Class II molecules.

Answer 174

An MHC Class II molecule has a binding cleft or groove for a 10-30 amino acid long peptide.

Answer 175

While in transit from the trans-golgi reticulum to the endocytic vesicles, the invariant chain is degraded, opening up the molecule to bind exogenous peptides.

Answer 176

The processed antigen meets up with class II in a specialized cellular compartment called the MIIC. The invariant chain is degraded from the class II molecule, leaving a peptide from the invariant chain occupying the peptide-binding site. This CLIP peptide (class II associated invariant peptide) is dissociated from the site by an HLA-DM catalysed process, and the antigenic peptide binds to the antigen-binding groove, and the complex is tranported to the cell surface.

Answer 177

The MHC Class II molecule with bound peptide interacts with CD4+ T cells and is the ligand for the CD4 T cell antigen receptor.

Answer 178

anchor residues.

Answer 179

Endogenously produced antigen, e.g. a viral protein, is processed and associates with MHC Class I molecules in the Rough Endoplasmic Reticulum.

Answer 180

Processing of endogenous antigens for Class I binding occurs in the cytoplasm in the proteosomes or immunoproteasomes, which are multicatalytic proteinase complexes.

Answer 181

At least two components of immunoproteasomes are encoded within the Class II MHC region, the LMP-2 and LMP-7, and account for substrate specificity, leading to generation of appropriate peptides for binding to MHC class I molecules. Peptides are imported from the cytoplasm across the ER membrane with the help of transporter proteins (TAP-1 and TAP-2). These are also encoded by Class II region of the MHC.

Answer 182

Peptide helps to stabilize Class I molecules - "Empty" Class I molecules rapidly dissociate and don’t get transported to the cell surface.

Answer 183

Peptides transported into ER bind preferentially to MHC class I for two reasons: 1) Peptide binding sites of class II are blocked by the invariant chain (Ii); 2) TAP is physically associated with class I, so that transported peptides are delivered directly to class I.

Answer 184

Compartmentalization (i.e. where it is found in the cell) of Ag determines the type of antigen presentation. The same antigen introduced differently can result in differential presentation.

Answer 185

An exception to the compartmentalization rule is the ability of DC, under some circumstances to “cross-present” antigen. Here, antigen is taken up by the DC and presented with Class II (the expected pathway) and “cross-presented” with MHC Class I to CD8 cells.

Answer 186

Cross-presentation allows DC to provide the appropriate signals 1 and 2 to CTL without having to be infected by a given pathogen (usually a virus). This lack of requirement for the DCs to be infectable by every virus ensures that virus cannot simply mutate (for example, by making themselves unable to infect the DC) to escape antigen presentation.

Answer 187

T cells continually survey APCs for foreign peptides. Although most MHC express self peptides, T cells are usually non-responsive to self-peptides.

Answer 188

Immunodominant peptides are those that: 1. Bind most avidly to MHC molecules. Some individuals may lack MHC-class II alleles that bind to a particular peptide. MHC polymorphism across a species is important here. - failure to respond to an antigen could be due either to lack of MHC class II capable of binding the peptide or to the lack of TCR that recognize the antigen. Note: The largest number of T cell clones will be directed to the immunodominant peptide(s).

Answer 189

CD1 and antigen presentation: CD1 molecules are non-polymorphic MHC-like molecules expressed by DC. They present lipids and some glycolipids to specialized cells called γ/δ T cells and NKT cells.

Answer 190

Superantigens: bind outside of the peptide binding site and don’t require processing. Turn on a broader range of T cells than conventional antigens.

Answer 191

The T cell receptor is made up of 2 chains, the α chain and the β chain, each is the product of a single germline gene. The B cell receptor is made up of a heavy chain and a light chain. There are 2 light chain , κ and λ. One or the other is included in the BCR expressed by an individual B cell.

Answer 192

1. Multiple copies of each gene segment that makes up the variable region joined in different combinations (combinatorial) 2. Addition and subtraction of nucleotides by the recombination process (junctional diversity) 3. Many different combinations of H and L chain V regions that pair to form the antigen binding site (combinatorial) 4. Somatic hypermutation occurring in the germinal center

Answer 193

When this occurs the B cell will attempt to rearrange the second H chain allele. Once this is done successfully, light chain gene rearrangement occurs, first the κ, then the λ, loci. Rearrangements continue until an IgM molecule made up of 2 heavy chains and 2 light chains is expressed on the cell surface.

Answer 194

H chain genes: Stem cell is germline. early pro D-J*, late pro V-DJ*, all cells up to matureB VDJ* L chain genes: Stem cell to large pre B cell is germline, small pre V-J*, immature and mature VJ* Mu chain transiently expressed as part of pre B cell receptor at large pre B cell (most of mu chain is intrancellular). Small pre mu chain intracellular, immature and mature IgM on cell surface; mature also has IgD. Note: IgM and IgD are made from alternatively spliced H chain transcripts.

Answer 195

Occurs at the somatic level not the germline. During the maturation process, B and T cell precursors express a unique set of enzymes that carry out these rearrangements. These include RAG1, RAG2 and Tdt (Terminal deoxynucleotidyl transferase). Other enzymes involved in this process are part of the DNA repair machinery (Ku70 and Ku80) found in most cell types.

Answer 196

DNA splicing occurs when RAG1 and 2 bind to the RSS located at the termini of D, J and V segments. After the DNA is cut, the cut ends are ligated imprecisely at the coding junction. In that process Tdt adds a variable number of nontemplated (or N) nucleotides thereby adding additional diversity to the Variable region of the BCR and TCR. The intervening DNA sequences are excised during this process.

Answer 197

Positive selection eliminate thymocytes that do not recognize self MHC - MHC restriction. Negative selection eliminate thymocytes that either bind to self MHC alone or to self MHC with self antigen with high affinity - self tolerance.

Answer 198

alpha and beta chains, CD4 and CD8 coreceptor molecules

Answer 199

Bone marrow derived lymphocyte progenitors develop into mature lymphocytes that populate peripheral lymphoid tissues.

Answer 200

MHC-restriction and self tolerant. Lacking self tolerance will lead to an attack on own tissue antigens and autoimmune diseases.

Answer 201

early maturation involving lineage commitment and proliferation, somatic recombination and expression of T cell antigen receptor genes, and selection and acquisition of the mature repertoires and functional capabilities.

Answer 202

1. Lymphocyte precursors arise in the bone marrow from the same pluripotent stem cells that give rise to other blood cells such as granulocytes, monocytes, erythrocytes and platelets 2. Precursors of T lymphocytes leave the bone marrow and migrate to the thymus, which is the major site of T cell maturation 3. Early T cell maturation is characterized by high mitotic activity, stimulated mainly by IL-7, resulting in marked increases in number.

Answer 203

Mutations in the IL-7 receptor γ chain result in X-linked severe combined immunodeficiency disease in humans. Disease is characterized by a block in T cell development.

Answer 204

1. Thymus is the major site of maturation of T cells 2. Lymphocyte precursors migrate from the bone marrow to the thymus. 3. Notch signaling instructs the precursor to commit to the T cell lineage rather than the B cell lineage. 4. Immature, developing lymphocytes in the thymus are called thymocytes.

Answer 205

5. Immature thymocytes are found in the cortex, mature cells are found in the medulla 6. Thymic environment provides stimuli that are required for the proliferation and maturation of thymocytes. 7. Thymic epithelial cells and bone marrow-derived dendritic cells play important roles in T cell maturation 8. The rates of cell proliferation and apoptotic death are extremely high in cortical thymocytes.

Answer 206

In DiGeorge Syndrome in humans, the thymus does not form and the affected individuals produce B cells but few T cells.

Answer 207

1. During T cell maturation, there is a precise order of somatic recombination of TCR genes and expression of the TCR and CD4 and CD8 coreceptors 2. Thymocytes with four phenotypes can be found:double negative (CD4 & CD8), double positive (CD4 & CD8), single positive (CD4) or (CD8). 3. Double positive (CD4 & CD8) thymocytes comprise the majority of the cells in the thymus 4. Expression of TCR αβ, CD4 and CD8 occurs prior to the start of any selection processes. 5. Several transcription factors have been identified that guide the development of thymocytes from one stage to the next.

Answer 208

1. Expression of antigen receptor genes is the key event in lymphocyte maturation required for the generation of a diverse repertoire and for the processes that ensure selective survival of lymphocytes with useful specificities 2. Functional antigen receptor genes are produced in immature T cells in the thymus by a process of somatic recombination, in which a set of germline DNA sequences that are initially separated from one another are brought together by enzymatic deletion of intervening DNA. 3. Antigen receptors deliver signals to developing lymphocytes that are required for the survival of these cells and for their proliferation and continued maturation.

Answer 209

1. The diverse antigen receptors of T lymphocytes are produced in immature cells from genes that are formed by somatic recombination of a limited number of germline gene segments. 2. The germline organization of TCR genetic loci are characterized by spatial segregation of sequences that must be joined together to produce functional genes coding for antigen receptor proteins

Answer 210

3. Genes encoding the TCR α, β and γ chains are in three separate loci whereas the TCR δ chain locus is found within the TCR α locus 4. Each germline TCR locus includes V, J and C gene segments and the TCR β and δ loci also have D segments. 5. At the 5’ end, there is a cluster of several V gene segments. 6. At varying distances 3’ of the V genes are the C gene segments. 7. There are J segments between the V and C genes in all TCR loci and there are D segments in the TCR β and δ chain loci.

Answer 211

8. Somatic recombination involves selecting one V gene, one J gene and one D gene in the developing lymphocytes and bringing these gene segments together to form a single V(D)J gene that will code for the variable region of the TCR. 9. Primary RNA is processed so that the V(D)J segment becomes contiguous with the C gene, forming an mRNA that is translated to produce one of the chains of the TCR. 10. Rearrangement of TCR gene segments is mediated by the coordinated activities of several enzymes that are called V(D)J recombinases.

Answer 212

1. After antigen receptors are expressed on immature lymphocytes, the cells with useful receptors are preserved and many potentially harmful, self-reactive cells are eliminated by selection processes induced by antigen receptor engagement. 2. Selection of developing T cells is stimulated by recognition of antigen (peptide-MHC complexes) in the thymus and is responsible for preserving useful cells and eliminating potentially harmful ones. 3. Positive selection – is the process that eliminates immature T cells that do not recognize self MHC or that recognize self MHC with a high avidity. Positive selection ensures maturation of T cells whose receptors bind with low avidity (weakly) to self MHC molecules. 4. Negative selection – is the process that eliminates developing lymphocytes whose antigen receptors bind strongly to self antigens

Answer 213

- Positive selection is the process in which thymocytes whose TCRs bind with low avidity (weakly) to self peptide-self MHC complexes are stimulated to survive - Positive selection ensures that the T cells that mature are self MHC-restricted. - Positive selection works by promoting the selective survival and expansion of thymocytes with self MHC-restricted TCRs and by permitting thymocytes whose TCRs are not self MHC-restricted to die by apoptosis.

Answer 214

4. Positive selection of T cells requires the recognition of self MHC molecules in the thymus by the TCRs of immature (double-positive) thymocytes. 5. During the transition from double-positive to single-positive cells, thymocytes with class I-restricted TCRs become CD8+CD4--, and cells with class II-restricted TCRs become CD4+CD8-.

Answer 215

6. Thymic epithelial cells are important in positive selection. 7. Peptides bound to MHC molecules on thymic epithelial cells play an essential role in positive selection

Answer 216

8. Negative selection is the process in which thymocytes whose TCRs bind strongly to self peptide antigens in association with self MHC molecules are deleted. 9. Negative selection of thymocytes works by inducing apoptotic death of cells whose receptors recognize abundant self peptide-MHC complexes in the thymus with high avidity. 10. Negative selection ensures that mature T lymphocytes are tolerant to self - Bone marrow-derived dendritic cells

Answer 217

12. AIRE- autoimmune regulator is expressed in the thymic medulla and promotes the expression of proteins normally expressed in peripheral tissues. 13. Mutations in AIRE give rise to human autoimmune disease – autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) or autoimmune polyglandular syndrome type I.

Answer 218

positive and negative selection is thought to hinge on the strength of self-peptide:MHC binding by the TCR. Low-affinity interactions rescue the cell from death by neglect, leading to positive selection; high-affinity interactions induce apoptosis and thus negative selection

Answer 219

- the presentation of peptide associated with MHC Class I or Class II, and the second is the delivery of the co-stimulatory signal from the antigen presenting cell to the T cell. This two signal requirement serves to prevent unnecessary (and potentially dangerous) activation of the immune response to self-antigens

Answer 220

T cells and other cells in the immune response receive activating or suppressive signals from cytokines, sometimes considered to be “signal 3”.

Answer 221

Naïve T cells that have not encountered antigen are resting (Go) RACE MD : recognition, activation, clonal expansion/differentiation, effector phase, memory, Decline of T cell responses

Answer 222

Recognition Phase | Antigen is presented to the CD4 T cell by APC in the presence of co-stimulating signal

Answer 223

Activation Phase Production of autocrine growth factors such as IL-2 leading to proliferation and differentiation. IL-2 binds to IL-2 receptor ----> cells move from G1 to S

Answer 224

Clonal expansion/Differentiation The T cells dramatically expand in response to IL-2 and other cytokines. Naïve T cells specific for a given antigen may represent as few as 0.0001% of the resting T cells whereas after expansion, antigen-specific CD8 cells may represent 10% of the CD8 pool or as much as 1% of the CD4 pool. T cells begin to divide within 18 hr after stimulation and each round of replication takes about 6 hr.

Answer 225

Effector Phase Cells carry out their functions (e.g., CD4 produce cytokines that lead to B cell activation for antibody production; macrophage activation; help for CTL; CD8 kill antigen bearing cells, produce cytokines).

Answer 226

Memory Cells specific for an antigen respond rapidly in secondary immune response. These antigen-specific memory cells may be approximately 0.01% of the circulating T cells. In neonates, a large majority of the T cells are naïve, whereas in adult humans, about half are memory.

Answer 227

Decline of T cell responses: Necessary to return the immune system to a state of homeostasis and rid the body of unnecessary T cells. Antigen-activated T cells die by apoptosis.

Answer 228

For example IL-2 is rapidly upregulated (45 min), followed by the IL-2 receptor (2 hours); some come up much later and allow trafficking of T cells to different parts of the body.

Answer 229

Naïve T cells have a higher threshold of activation than activated or memory T cells. Naïve T cells are primarily activated in secondary lymphoid organs (i.e. the lymph node) through dendritic cells that provide strong co-stimulation. In contrast, activated T cells and memory cells can respond to pathogens in periphery and are less dependent upon co-stimulation

Answer 230

T cell receptor: has a short cytoplamic domain with no enzymatic activity; consists of an α/β or γ/δ heterodimer which recognizes antigen.

Answer 231

Complexed with six invariant chains: CD3γ, CD3δ, two copies of Cd3ε and a ζ homodimer. These chains contain 1-3 copies of a conserved amino acid motif of pairs of tyrosines (Y) and leucine (L), called the “immunoreceptor tyrosine-based activation motif” (ITAM). This motif has signaling function. Other receptors of the immune system have similar structures; each of these are multi-chained and have one or more ITAMs associated with at least one chain.

Answer 232

Peptide - MHC complex on APC (monocytes/macrophages, dendritic cells, B cells for activation of CD4 cells)

Answer 233

Accessory molecules: CD4, CD8, CD2, LFAs (Cell adhesion molecules), CD45 on T cells and correspondng ligands on APC.

Answer 234

Co-stimulatory molecules: CD80-(B7-1), CD86(B7-2) on APC; CD28 on T cells. Failure to co-stimulate will lead to anergy, which is the inability of the T cell to respond to activating stimuli.

Answer 235

A. TCR on resting T cell binds MHC/peptide on APC B. If APC had been activated by infection, then B7 (CD80 and CD86) is expressed on macrophage or B cell. Dendritic cells are the only cells that constitutively express B7 (and can be rapidly further upregulated), so they are most efficient at activating naïve T cells. This can be mimicked experimentally with substances that cross-link. e.g. anti-CD3 plus anti- CD28 will yield polyclonal T cell activation and proliferation.

Answer 236

C. CD80 and CD86 interact with CD28 on T cells ---> deliver costimulatory signal Why is this co-stimulation necessary? Without costimulation, need many more MHC-Ag/TCR interactions to activate cell. Failure to deliver this actually turns off the cell - anergy. Costimulation leads to upregulation of IL-2 production due to increased transcription and increased message stability. In addition, CD28 signaling is anti-apoptotic (pro-survival). Once the T cell is activated, co-stimulation is no longer required. For example, an activated cytotoxic T cell can kill a virally infected target that does not express B7. D. Role of CD40 in T cell activation. CD40 (on APC) interacts with CD40L (on T cells)  enchanced T cell activation, upregulation of B7 costimulation

Answer 237

This clustering of receptors is very important for the activation and is called the “immunological synapse”. This results in the rapid tyrosine phosphorylation of zeta chain of TCR and CD3 by tyrosine kinases lck, fyn and ZAP70 kinases. Tyrosine phosphorylation leads to creation of docking site for phospholipase C-2 (PLC). PLC itself is rapidly phosphorylated, leading to activation of the enzyme.

Answer 238

CD45; a tyrosine phosphatase is also involved in T cell activation - may function to remove negatively regulating tyrosine-P from lck and fyn

Answer 239

a. Immediate genes: require no protein synthesis b. Early: transcribed prior to mitosis c. Late: transcribed after mitosis a. Cellular proto-oncogenes/transcription factor genes: cfos, c-myc b. cytokine genes: e.g. IL-2 c. cytokine receptor genes: e.g. IL-2R

Answer 240

Cyclosporin A and FK506 inhibit signal transduction, inhibiting IL-2 (and other cytokines) production. These drugs block calcineurin activation of NFAT, which is required for transcription of the IL-2 gene.

Answer 241

ZAP-70 kinase deficiency leads to a form of severe combined immunodeficiency (SCID) by interfering with T cell activation.

Answer 242

Bruton's (X-linked) agammaglobulinemia, which is characterized by the patients having no serum Igs, is caused by a deficiency in a tyrosine kinase - Bruton's tyrosine kinase.

Answer 243

X-linked immunoproliferative disease is caused by deficiency in an adaptor molecule that binds to a cell surface protein of T and B cells. This leads to susceptibiltiy to viral infections, especially severe EBV.

Answer 244

X-linked hyper-IgM syndrome is caused by a mutation in CD40L expression on T cells.

Answer 245

Generically, cytokine binds to receptor, allowing dimerization or multimerization of receptor polypeptides at cell surface. This will cause activation of intracellular signaling pathways (e.g. kinase cascades), resulting in the production of transcription factors that migrate to the nucleus and bind to enhancer regions of genes induced by that cytokine.

Answer 246

the IFN alpha receptor has associated with it two JAK kinases, Jak-1 and Tyk2, which then phosphorylate Stat 1 and Stat 2. These two transcription factors form a complex with a DNA-binding protein, p48, and translocate to the nucleus where they induce transcription of genes with an interferon response element.

Answer 247

IL-2Rγ chain deficiency. The γ chain is a shared receptor chain - used by IL-2, -4, -7, -9, -11 and -15. Of these, the inability to signal through IL-7 is the most important for the SCID phenotype.

Answer 248

Mucosal surfaces are continually exposed to a large number of foreign antigens, including benign antigens from the environment, such as food and microorganisms, as well as harmful antigens such as external infectious agents.

Answer 249

gastrointestinal tract, respiratory | tract, salivary glands, lacrimal glands, mammary glands, and genito-urinary tract.

Answer 250

the mucosal surfaces of the human body represent an area 100 times greater that of the skin. mucosal immune system

Answer 251

more than 1000 commensal bacteria which are present in the colon and lower ileum

Answer 252

inductive site, leading to an immune response at multiple effector sites.

Answer 253

1. Gastrointestinal tract – gut associated lymphoid tissue (GALT) 2. Respiratory tract – bronchial associated lymphoid tissue (BALT) 3. Nasal associated lymphoid tissue (NALT) 4. Genitourinary tract 5. Lacrimal glands 6. Salivary glands 7. Mammary glands

Answer 254

1. The administration of antigen at one mucosal site results in specific antibody production at distant mucosal sites. 2. B cells in the mucosa are selectively induced to produce dimeric IgA rather than other isotypes. The selective switch of B cells to IgA is believed to be mediated by specific cytokines produced by T cells in the inductive sites. 3. Conventional T cells, particularly CTLs, are also an important component of the mucosal immune response. 4. Induction of a response via a mucosal site generally elicits a systemic immune response as well, such that serum antibodies can be detected. This indicates that a mucosal encounter with antigen generates subsets of T and B cells that home to mucosal sites and also to spleen and regional nodes.

Answer 255

Mechanistically, the induction process can be divided into the following steps: 1. Antigens entering the digestive tract are taken up by specialized mucosal cells called M cells. M cells internalize the antigen and transport it across the epithelium where antigen can be taken up by APCs such as dendritic cells (DC). “M” cells are formed in mucosal epithelium in response to signals from lymphocytes. 2. Antigen can be taken up by DC that have dendrites extending through the epithelial tight junction into the lumen. Antigens are then presented to lymphocytes (in the intestine, these are located in Peyer’s patches). 3. Lymphocytes (both B and T cells) leave the mucosal site and travel to the mesenteric lymph nodes, then into the lymph. 4. Via the thoracic duct, the lymphocytes exit the lymph and enter the circulation. 5. Circulating lymphocytes “home” to positions within the mucosal lamina propria throughout the body, including sites distant from the original antigenic encounter. The homing of lymphocytes to mucosal sites involves specific interactions of both adhesion molecules and chemokine. 6. B Lymphocytes within the peripheral tissues proliferate and differentiate into IgA secreting plasma cells at effector sites.

Answer 256

1. Peyer’s patches Aggregates of lymphoid cells with B cell follicles & smaller T cell areas 2. Lymphoid follicles Smaller; mainly B cells. Also found in respiratory tract (BALT), lining of nose (NALT)- together referred to as MALT mucosa associated 3. Appendix (Tonsils, adenoids) 4. Mesenteric nodes Peyer’s patches & lymphoid follicles connected by lymphatics to draining nodes Peyer’s patches, mesenteric nodes differentiate independently of systemic immune system during fetal development (control chemokines)

Answer 257

1. Epithelial cells covering Peyers Patches 2. Differ from epithelial cells (enterocytes) No microvilli; broader microfolds Do not secrete enzymes, mucus, and no thick surface glycocalyx

Answer 258

1. Transport organisms from gut lumen to immune cells across epithelial barrier 2. Endocytose or phagocytose Ag at AP surface & deliver it to DCs or T cells via transcytosis at BL surface 3. Note: some pathogens (Shigella, Salmonella, Yersinia) exploit M cells as a way to penetrate the intestinal epithelium

Answer 259

1. DCs recruited to the mucosa in response to chemokine constitutively expressed by epithelial cells 2. Extend processes across epithelium to capture Ag in lumen 3. DCs also prevalent within lamina propria. CCL20 (MIP-3α) & CCL9 (MIP-1γ) bind to receptors on DCs (CCR6 and CCR1, respectively). Ag loaded DC migrate from dome region of Peyer’s patch to T cell area or to draining lymphatics to mesenteric nodes

Answer 260

1. Resident T cells found in epithelium and lamina propria 2. Epithelium contains mostly CD8 T cells, whereas lamina propria is more heterogenous (CD4, CD8, plasma cells, macrophages, DCs, eosinophils and mast cells). In intestine and respiratory tract, plasma cells predominately IgA. In genital tract: IgG> IgA 3. Neutrophils are found usually only in response to inflammation/infection 4. T cells in lamina propria of small intestine express integrin α4β7 and CCR9, which attracts them into the tissue from bloodstream; 5. Epithelial cell T cells express integrin αEβ7, which binds to E-cahedrin on epithelial cells.

Answer 261

2. Enter Peyer’s patches (or nodes) through endothelial venules directed by homing receptors, CCR7 & L-selectin 3. If no Ag is encountered, exit via efferent lymphatics & return to bloodstream 4. If Ag is encountered, cells become activated, exit via lymph nodes to thoracic duct & recruited back to gut 5. T cells that first encounter Ag in GALT express gut-specific homing receptors (α4β7 and CCR9)

Answer 262

1. Expression of homing receptors triggered by GALT DC 2. α4β7 binds to the mucosal vascular addressin (MAdCAM-1) expressed on gut endothelial cells 3. CCR9 binds to CCL25(TECK) on gut epithelium (small intestine) 4. Priming explains why vaccination by mucosal route against intestinal infections (e.g. Rotavirus) ensures imprinting to the gut. 5. [YES] MAdCAM-1 also expressed in other mucosal sites: T cells primed in GALT can recirculate as effector cells to respiratory, genital or lactating breast tissue: “common mucosal immune system” 6. Vaccines: Mucosal route can be used to protect multiple mucosal sites

Answer 263

1. Dominant class in gut & respiratory tract (not genital tract) 2. In blood, IgA mostly monomer (IgA1:IgA2=10:1) 3. In mucosa, dimer linked by J chain (IgA1: IgA2=3:2) 4. Class switching from IgM to IgA producing cells occurs in response to TGFβ 5. Common intestinal pathogens can cleave IgA1. IgA2 more resistant

Answer 264

1. Activated B cells (like T cells) express homing integrin (α4β7) & CCR9/10, which localizes them to gut 2. IgA producing plasma cells secrete IgA dimers, bind to poly-Ig receptor expressed on BL surface of immature epithelial cells at base of intestinal crypts 3. Bound complex taken up by cells; traversed to AP surface by transcytosis 4. Poly-Ig receptor cleaved releasing IgA dimer & secretory component at luminal surface; secretory IgA 5. IgA binds to mucins at epithelial surface via carbohydrates on secretory component; Retention of IgA at epithelial surface prevents adherence of microbes & neutralizes toxins, etc.

Answer 265

1. IgA does not activate complement pathway 2. Does not trigger inflammatory response 3. Restricts commensal flora to the lumen

Answer 266

1. Common: 1:500-1:700 in Caucasian population 2. Most individuals have no clinical problems 3. Associated with IgG2 subclass deficiency→↑ risk infections 4. IgM may replace IgA in secretions; IgM also is J-chain linked & binds poly-Ig receptor. IgM-producing plasma cells are ↑ in IgA deficiency

Answer 267

1. Most T cells in lamina propria CD45RO+ (similar to effector/memory T cells) Express gut homing markers Express receptors for inflammatory chemokines e.g CCL5 (RANTES) 2. Proliferate poorly in response to Ag or mitogens 3. Secrete large amounts of cytokines (IL-10, IL-5 and IFN-γ constitutively 4. Function in healthy gut uncertain could be a regulatory role

Answer 268

A distinct population of lymphocytes, mostly CD8+ T cells are found in the gut epithelium. a. 10-15 lymphocytes/100 epithelial cells b. 90% are T cells; 80% CD8+ c. Express homing markers CCR9 &αEβ7; binds E cadherein on epithelial cells d. Activated e. + perforin and granzyme in intracellular granules f. Relatively restricted use of V(D)J gene segments; responsive to limited Ag repertoire

Answer 269

a. Type A: conventional CD8 cytotoxic effectors MHC-restricted express CD8α:β b. Type B: i. Express CD8α:α ii. Express NKG2D (activating C-type lectin NK receptor) which binds to 2 MHC-like-molecules; MIC-A, MIC-B that are expressed on epithelial cells in response to stress/damage & killed via perforin/granzyme pathway iii. Activation of these IEL cells mediated by IL-15 ↑in celiac disease

Answer 270

1. Mucosal surfaces are not sterile 2. Mucosal immune system must differentiate harmless (endogenous flora) from pathogenic microbes and respond differently 3. Gut is most frequent site of infection

Answer 271

1. Mucosal epithelial cells are polarized 2. Apical surface faces the intestinal lumen; BL surface faces the adjacent epithelial cells and underlying basement membrane 3. Polarized expression of different receptors/proteins/channels 4. EXPRESS Toll-like Receptors (TLRs) at both membranes, but responses differ 5. Activation by commensal bacteria has an essential role in maintaining colonic homeostasis

Answer 272

1. TLRs within intracellular vesicles 2. NOD1/NOD2 (nucleotide-binding oligomerization domain). NOD1 recognizes muramyl tripeptide on GNR. NOD2 recognizes muramyl dipeptide in peptidoglycan of most bacteria Signaling activates NFkB pathways Chemokines, cytokines, defense's Activation of signaling pathways: double edged sword Facilitate further invasion (e.g. IL-1β and TNFα disrupt tight junctions Inflammation causes symptoms, but also recruits immune cells & initiates adaptive immune response to eliminate microbe

Answer 273

1. Adhere to M cells, cause apoptosis of M cell, infect macrophages and epithelial cells; trigger TLR5(flagellin) at BL membrane and trigger NFkB inflammatory pathways 2. Invade by direct adherence of fimbriae to luminal epithelial surface 3. Enter DCs that sample gut luminal contents

Answer 274

2. Compete with pathogenic bacteria prevent them from colonizing & invading 3. Directly inhibit proinflammatory signaling pathways 4. TLR response to commensals controls inflammation 5. Loss of normal gut flora (i.e. in response to antibiotics) allows other bacteria to grow: C difficile 6. Integrity of intestinal epithelium disrupted (trauma, infection, vascular disease) 7. Nonpathogenic commensal invade blood stream→disease

Answer 275

2. sIgA and T cells recognize commensals. Effect responses not typically elicited. Do not typically invade: compartmentalized response 3. Animals raised in germfree (gnotobiotic) environment Reduced size of lymphoid organs Low Ig levels Reduced immune responses

Answer 276

2. Activate B cells into IgA producers - redistributed to lamina propria 3. Epithelial cells produce TGFβ, TSLP, PGE2- maintain DCs in quiescent state 4. When present Ag to naive T cells, generate Treg response (anti-inflammatory) 5. Commensals do not penetrate intact epithelium, do not activate NFkB, lack virulence factors 6. If regulatory mechanisms fail, systemic immune responses generated (TH1)→disease

Answer 277

2. DCs now become fully activated and present Ag to T cells to generate effector T cell response 3. Both DC populations – inflammatory and regulatory may exist simultaneously: state of physiological inflammation 4. Hygiene hypothesis: absence of exposure to helminthes and other Ags results in hypersensitivity responses to harmless environmental Ags and increased autoantigen responses

Answer 278

Summary for personal wrap up. 10, 12 lack co-stimulatory molecules to activate naïve T cells into effectors

Answer 279

Once the H chain is successfully rearranged and a functional protein is expressed This allows multiple light chain genes to be paired with a single heavy chain gene, again increasing diversity.

Answer 280

are eliminated if they are stimulated by cell surface self-antigens, made anergic if they are stimulated by soluble self-antigens. same rearranged V region, different C region) enter the periphery.

Answer 281

secondary lymphoid organs.

Answer 282

Activation requires BOTH BCR crosslinking by antigen, AND T cell help. The exception is T-independent antigens, which require only BCR crosslinking.

Answer 283

T cell help comes from activated T cells, which recognize a peptide derived from the same (or a linked) antigen seen by the B cell. When the BCR binds its target antigen (eg. a surface protein on a virus particle), it also internalizes that antigen (virus particle in this example) and presents peptides derived from it (all viral proteins). Any T cell recognizing any peptide derived from any of the viral proteins will bind to this peptide when it is presented on MHC Class II molecules on the B cell surface. T cell help consists of CD40-CD40L interaction, as well as cytokines produced by the activated T cell. T cell help activates the B cell to proliferate. • Activated B cells can go one of 2 ways. 1. Begin to differentiate into IgM-producing plasma cells 2. Move into a germinal center

Answer 284

In the germinal center, B cells undergo affinity maturation by means of somatic hypermutation, as well as class switching. Somatic hypermutation promotes the emergence of B cells with the highest affinity for the antigen. There will be multiple B cell clones with BCR V regions recognizing different pieces of the same antigen that enter the germinal center, but only a few will remain as this affinity maturation process progresses. Affinity maturation leads to preferential expansion of the B cell clones with the highest affinity BCRs. Also in the germinal center, class switching occurs, with elimination of the μ and δ C region gene segments.

Answer 285

Plasma cells arising from the germinal center secrete antibodies with high affinity binding to antigen, expressing the constant region genes for γ (IgG), α (IgA) or ε (IgE). Memory B cells with the same specificity and C region return to the bone marrow and can be activated at a later time.

Answer 286

T-independent antigens are usually bacterial components. These antigens cancactivate B cells without T cell help. The antibodies produced by B cells activated in this way are generally low affinity and of the IgM class.

Answer 287

B cells in the germinal center express the enzyme AID (activation-induced cytidine deaminase) that introduces point mutations into the rearranged H and L chain V region genes AND plays a role in class switching. Most of the point mutations will decrease BCR affinity, and cells with BCRs that no longer bind antigen with high affinity are eliminated. When mutations increase BCR affinity for antigen binding, these will continue to proliferate with T cell help. This process goes through many cycles. Class switching.

Answer 288

Class switch requires RNA transcription through the Switch region adjacent to the new constant region module as well as through the μ constant region genes. Which non-μ constant region gene is chosen for transcription depends upon the cytokine levels present as the B cell matures. These breaks are recognized by DNA repair enzymes, which bring together the Switch region preceding μ with the switch region preceding the γ1, γ2, γ3, γ4, α1, α2 or ε constant region. Recombination through the switch region occurs, with excision of the intervening DNA.

Answer 289

Antibodies are very important for neutralizing extracellular pathogens and toxins. When a pathogen or toxin is coated by antibody it cannot bind its cell surface receptor and cannot infect. Antibodies can coat pathogens, promoting phagocytosis when the constant region of the antibody binds to Fc receptors on the phagocyte. Antibodies (IgM and IgG) can activate complement. Complement activation can lyse some bacterial, but activated complement also enhances opsonization. Many of these functions depend upon the Isotype, or the constant region

Answer 290

neutralization- (+++) IgG1-4, IgA ; (+) IgM opsonization- (+++) IgG1 ; (++) IgG3 ; (+) IgG4, IgA, IgM sensitization for killing by NK cells- (++) IgG1/3 sensitization of mast cells- (+++) IgE ; (+) IgG1/3 activates complement system- (+++) IgM, IgG3 ; (++) IgG1 ; (+) IgG2, IgA

Answer 291

Transport across epithelium- (+++) IgA dimer ; (+) IgM Transport across placenta- (+++) IgG1 ; (++) IgG3 ; (+) IgG2 ; (+/-) IgG4 Diffusion into extravascular sites- (+++) IgG1-4 ; (++) IgA monomer ; (+) IgE ; IgM (+/-) Mean serum level (mg/ml)- (9) IgG1 ; (3) IgG2 ; (1) IgG3 ; (0.5) IgG4 ; (1.5) IgM ; (2.1) IgA ; (0.04) IgM ; (3 x 10^-5) IgE multimers, dimers, pentamers

Answer 292

The activation and clonal expansion of a naïve T cell on its initial encounter with antigen is refered to as priming to distinguish it from the responses of effector T cells to antigen on their target cells and the responses of primed memory T cells.

Answer 293

the activation of naïve T cells is controlled by three kinds of signals: 1) MHC:TCR; 2) B7:CD28; 3) Cytokines:cytokine receptor. Variation in signal 3 causes naive CD4 T cells to differentiate into distinct subsets with distinct types of effector functions.

Answer 294

``` T helper (Th)1 Th2 Th17 Tfh Treg. ```

Answer 295

intracellular destruction of infected cells by cytotoxic CD8 T cells, or the destruction of intracellular pathogens in macrophages activated by CD4 TH1 cells.

Answer 296

CD4 TH17 cells help to recruit neutrophils to sites of infection early in the adaptive immune response, which is also a response aimed mainly at extracellular pathogens.

Answer 297

CD4 TH2 cells induce the switch to production of IgE antibodies, which are involved in the activation of effector responses aimed against extracellular multicellular parasites such as helminth worms.

Answer 298

``` TFH cells contribute to humoral immunity by stimulating the production of antibodies by B cells and inducing class switching, and can produce cytokines characteristic of either TH1 or TH2 cells. ```

Answer 299

Regulatory T cells tend to suppress the adaptive immune response and are important in preventing immune responses from becoming uncontrolled and in preventing autoimmunity.

Answer 300

Uptake of antigens into the endocytic systemby dendritic cells, either by receptor-mediated phagocytosis or by macropinocytosis, is considered to be the major route for delivering peptides to MHC class II molecules for presentation to CD4 T cells. Production of antigens in the cytosol, for example as a result of viral infection, is thought to be the major route for delivering peptides to MHC class I molecules for presentation to CD8 T cells. It is possible, however, for exogenous antigens taken into the endocytic pathway to be delivered into the cytosol for eventual delivery to MHC class I molecules for presentation to CD8 T cells, a process called cross-presentation. Finally, it seems that antigens can be transmitted from one dendritic cell to another, particularly for presentation to CD8 T cells.

Answer 301

Dendritic cells take up antigens at a site of wounding and infection and carry them to the draining lymph node for presentation to naive T cells.

Answer 302

Immature dendritic cells are specialized in the uptake of pathogens and their antigens. On migration to the lymph node, they settle in the T-cell areas and differentiate into mature dendritic cells that are specialized in activating naive T cells.

Answer 303

Dendritic cells mature through at least two definable stages to become potent antigen presenting cells in peripheral lymphoid tissue.

Answer 304

Immature dendritic cells originate from bone marrow progenitors and migrate via the blood, from which they enter and populate most tissues, including some direct entry into peripheral lymphoid tissues.

Answer 305

Entry to particular tissues is based on chemokine receptors they express. Immature dendritic cells in tissues are highly phagocytic via receptors such as dectin-1, DEC 205, DC-SIGN, and langerin, and are actively macropinocytic, but they do not express costimulatory molecules. They carry most of the different types of Toll-like receptors (TLRs).

Answer 306

At sites of infection, immature dendritic cells are exposed to pathogens, leading to activation of their TLRs. TLR signaling causes the dendritic cells to become licensed and begin to undergo maturation, which involves induction of the chemokine receptor CCR7. TLR signaling also increases the processing of antigens taken up into phagosomes. Dendritic cells expressing CCR7 are sensitive to CCL19 and CCL21, which directs them to the draining lymphoid tissues. CCL19 and CCL21 provide further maturation signals, which result in higher levels of co-stimulatory B7 molecules and MHC molecules.

Answer 307

In the draining lymph node, mature conventional dendritic cells have become powerful activators of naive T cells but are no longer phagocytic. They express B7.1, B7.2, and high levels of MHC class I and class II molecules, as well as high levels of the adhesion molecules ICAM-1, ICAM-2, LFA-1, and CD58.

Answer 308

Antigen presentation by dendritic cells is primarily involved in activating naive T cells for expansion and differentiation. Macrophages and B cells present antigen primarily to receive specific help from effector T cells in the form of cytokines or surface molecules.

Answer 309

Naive T cells recirculate through secondary lymphoid organs, such as the lymph node. They leave the blood at high endothelial venules and enter the lymph node cortex, where they mingle with professional antigen-presenting cells (mainly dendritic cells and macrophages). T cells that do not encounter their specific antigen leave the lymph node in the efferent lymph and eventually rejoin the bloodstream. T cells that encounter antigen on antigen-presenting cells are activated to proliferate and to differentiate into effector cells. These effector T cells can also leave the lymph node in the efferent lymph and enter the circulation.

Answer 310

Pathogen-specific T cells in the blood enter a lymph node that is draining an infected tissue. They encounter pathogen antigens, are activated, and leave as effector cells in the efferent lymphatic. At the same time, other pathogen-specific T cells in the blood enter an ‘upstream’ lymph node that is draining healthy tissue. They do not encounter their antigen there but can be carried to the infected lymph node via a connecting lymphatic vessel. There, they too will become activated by dendritic cells presenting pathogen antigens.

Answer 311

The recruitment of T-cells into lymph nodes is guided by numerous receptor–ligand interactions. Selectins expressed on the surface of the T-cell transiently interact with their ligands expressed on high endothelial venules and mediate T-cell rolling along the endothelium. The decrease in T-cell velocity subsequently permits the T-cell to detect the chemokine CCL21 immobilized on the surface of the endothelium via the CCR7 chemokine receptor. Signalling through the CCR7 receptor on T-cells promotes the activation of the LFA-1 integrin by a process known as ‘insideout’ activation. This increases the ligand-binding capacity of LFA-1 and therefore its ability to interact with ICAM-1 on the endothelial wall. The LFA-1/ICAM-1 interaction promotes firm adhesion of the T-cell to the vessel wall. Signalling through LFA-1 on T-cells via ICAM-1 also promotes cytoskeletal polymerization and remodelling, leading to the formation of a polarized T-cell and the initiation of T-cell migration. Chemokine and integrin-mediated signalling guides the T-cell along the endothelium in search of a suitable entry site into the lymph node (transmigration). Once the T-cell enters into the lymph node, chemokines such as CCL21 promote rapid T-cell motility and the scanning of antigen-presenting cells for cognate antigen.

Answer 312

Binding of the foreign-peptide:self-MHC complex by the T-cell receptor and a CD4 or CD8 coreceptor, transmits a signal to the T cell that antigen has been encountered (Signal 1). Effective activation of naive T cells requires a second signal, the co-stimulatory signal, to be delivered by the same antigen presenting cell (APC). CD28 on the T cell encountering B7 molecules on the antigen-presenting cell delivers signal 2, whose net effect is the increased survival and proliferation of the T cell that has received signal 1. For CD4 T cells in particular, different pathways of differentiation produce subsets of effector T cells that carry out different effector responses, depending on the nature of a third signal (Signal 3) delivered by the antigen presenting cell. Cytokines are commonly, but not exclusively, involved in directing this differentiation.

Answer 313

Naive T cells express CD28, which delivers a co-stimulatory signal on binding B7 molecules, driving their survival and expansion. Activated T cells express increased levels of CTLA-4. CTLA-4 has a higher affinity than CD28 for B7 molecules and thus binds most or all of the B7 molecules, serving to regulate the proliferative phase of the response.

Answer 314

Abatacept is the first therapeutic agent approved for treatment of rheumatoid arthritis that acts by selectively blocking the activation of T cells by disrupting costimulatory signals. Abatacept is a fusion protein formed by the extracellular CTLA-4 domain expressed on the T lymphocyte and a modified Fc fragment of human IgG1, which modulates costimulation when the antigen presenting cell comes into contact with the T lymphocyte.

Answer 315

Antigen-presenting cells, principally dendritic cells, provide signal 3 in the form of various cytokines or express surface proteins that induce the development of CD4 T cells into distinct types of effector cells. The environmental conditions, such as the exposure to various pathogens, determine which signal the antigen presenting cell will produce. When pathogens are absent, an abundance of transforming growth factor-β (TGF-β) and the lack of IL-6, IFN-γ, and IL-12 favor the development of FoxP3-expressing Treg cells. Early in infection, IL-6 produced by dendritic cells acts with TGF-β to induce TH17 cells expressing the transcription factor RORγT, which are amplified by IL-23. TFH cells, which require IL-6 and the transcription factor Bcl-6 for their function, provide help to B cells in the form of cytokines such as IL-21, and the surface molecule ICOS. Later, dendritic cells and other antigen presenting cells produce cytokines that promote either TH1 (IFN-γ and IL-12) or TH2 (IL-4) and suppress TH17 development. TH1 and TH2 cells express the T-bet and GATA3 transcription factors, respectively.

Answer 316

TH1 cells produce cytokines that activate macrophages, enabling them to destroy intracellular microorganisms more efficiently. TH2 cells produce cytokines that recruit and activate eosinophils, mast cells, and basophils, and promote barrier immunity at mucosal surfaces. TH17 cells secrete IL-17-family cytokines that induce local epithelial and stromal cells to produce chemokines that recruit neutrophils to sites of infection early in the adaptive immune response. TFH cells are a subset that localizes in B-cell follicles, but that produce cytokines characteristic of other subsets. TFH cells producing IFN-γ activate B cells to produce strongly opsonizing antibodies belonging to certain IgG subclasses (IgG1 and IgG3 in humans, and their homologs IgG2a and IgG2b in the mouse). Those TFH cells producing IL-4 drive B cells to differentiate and produce immunoglobulins of other types, especially IgE. ``` Regulatory T cells are a heterogeneous class of cells that suppress T-cell activity and help prevent the development of autoimmunity during immune responses. ```

Answer 317

TH1 cells secrete IFN alpha that activates macrophages to become highly microbicidal. Activated macrophages undergo changes that greatly increase their antimicrobial effectiveness and amplify the immune response.

Answer 318

Activated T cells proliferate and migrate into the joint. In the synovial tissue, T cells produce interferon-γ and other proinflammatory cytokines that stimulate macrophages and fibroblasts as well as chondrocytes, osteoclasts, and B cells. Activated macrophages and fibroblasts release a variety of cytokines, including TNF. TNF is a central component in the cascade of cytokines, stimulating the production of additional inflammatory mediators and the further recruitment of immune and inflammatory cells into the joint. Infliximab and adalimumab are monoclonal anti- TNF antibodies that bind to TNF with high affinity and prevent it from binding to its receptors. Etanercept is a fusion protein consisting of two p75 TNF receptors that are linked to the Fc portion of human IgG1. It also binds to TNF and prevents it from interacting with its receptors on cell surfaces.

Answer 319

Production of neutralizing antibodies; Production of IgE; Suppression of macrophages activation; Eosinophil activation.

Answer 320

Infection-induced interleukin (IL)-17 can be produced by several immune cells found in mucosal sites. A critical likely target of IL-17 is the mucosal epithelium, in which IL-17 augments granulocyte colony-stimulating factor (G-CSF) and CXC chemokine production, resulting in the recruitment of neutrophils that contribute to bacterial and fungal clearance at mucosal sites. IL-17 along with IL-22 also augments antimicrobial peptides and epithelial repair function important for control of extracellular fungal pathogens. In the setting of vaccine-induced immunity, Th17 cells can augment the recruitment of interferon- (IFN-)-producing Th1 cells to control intracellular pathogen growth.

Answer 321

Interleukin-17 (IL-17) acts on various cellular targets, leading to cell activation. The effect of IL- 17 on endothelial cells leads to inflammation and procoagulant activity. When acting on epithelial cells and fibroblasts, IL-17 leads to cytokine and enzyme production. On monocytes and dendritic cells, IL-17 contributes to inflammation by increasing the production of proinflammatory cytokines. In the context of joint inflammation, a process that involves osteoblasts and chondrocytes, IL-17 activates matrix destruction in cartilage and bone.

Answer 322

The interleukin-17 (IL-17)–T helper 17 (TH17) pathway is a rich source of targets for inflammatory disease drugs. The most advanced are Amgen's brodalumab, Lilly's ixekizumab and Novartis's secukinumab.

Answer 323

The development of most antibody-producing B cells occurs in the specialized germinal centre (GC) areas of the lymph nodes. Cells destined to become TFH cells acquire a CXCR5+CCR7- phenotype that results in their follicular positioning. The encounter between TFH cells and B cells can initiate the germinal center reaction and the formation of antibody-secreting plasma cells and memory B cells.

Answer 324

Upon antigen-specific activation by dendritic cells (DCs), CD4+CD25+ regulatory T (Treg) cells can suppress immune responses through (a) preferential consumption of IL-2 by CD4+CD25+ regulatory T cells instead of effector T cells, (b) induction of effector T cell apoptosis via CD30/CD30L interactions or perforin/granzyme B (GrB), (c) production of immunosuppressive cytokines IL-10 and TGF-β by CD4+CD25+ regulatory T cells (d) production of immunosuppressive tryptophane metabolites as a result of upregulation of indoleamine-2,3-dioxygenase (IDO) in DC.

Answer 325

Direct and Indirect: Naive CD8 T cell can be activated directly by a virus-infected dendritic cell. ``` Dendritic cell (or other virus-infected cell expressing MHC class II molecules) that induces insufficient costimulation can be helped by CD4 effector T cells to activate naive virus-specific CD8 T cells. ``` In this situation, the IL-2 secreted by the CD4 T cell acts directly on the naive CD8 T cell interacting with the same dendritic cell, and provides the necessary boost for activating the CD8 T cell.

Answer 326

``` CD8 cytotoxic cells kill target cells that display peptide fragments of cytosolic pathogens, most notably viruses, bound to MHC class I molecules at the cell surface. ``` CD8 cytotoxic T cells are specialized to kill cells infected with intracellular pathogens. The cellular polarization of CD8 T cells during specific antigen recognition allows effector molecules to be focused on the antigenbearing target cell. Cytotoxic T cells contain specialized lysosomes called cytotoxic granules, which contain cytotoxic proteins. INITIAL binding to a target cell through adhesion molecules does not have any effect on the location of the cytotoxic granules. Binding of the T-cell receptor causes the T cell to become polarized: reorganization within the cortical actin cytoskeleton at the site of contact aligns the microtubule-organizing center (MTOC), which in turn aligns the secretory apparatus, including the Golgi apparatus (GA), toward the target cell. Proteins stored in cytotoxic granules derived from the Golgi are then directed specifically onto the target cell. Cytotoxic CD8 T cells can kill several infected target cells in succession. Cytotoxic T cells kill target cells bearing specific antigen while sparing neighboring uninfected cells. Perforin, Granzyme and Granulusin are the cytotoxic effector proteins released by cytotoxic T cells.

Answer 327

Just for note.

Answer 328

Types I-III are antibody-mediated and are distinguished by the different types of antigens recognized and the different classes of antibody involved. Type I responses are IgEmediated. Mast cells are the major effector cells in Type I Types II and III are IgG-mediated. Complement-mediated and phagocytic effector mechanisms are involved in Type II and III responses. Type IV reaction is mediated by T cells.

Answer 329

1. More commonly known as allergy or immediate hypersensitivity. 2. Induced by certain type of antigens, referred to as allergens. 3. Antigen is exogenous. 4. Allergen induces IgE response. 5. Response time is immediate,15-20 minutes. 6. Appearance-Wheal and flare. 7. Histology-mast cells, basophils and eosinophils. 8. Mechanism-Initiated by binding of allergen to IgE on sensitized mast cells. 9. Examples of diseases-allergic rhinitis, asthma, systemic anaphylaxis. 10. Diagnostic tests-i) skin testing; ii) RAST. 11. Treatment-i) Hyposensitization; ii) anti-histamines; iii) Cromolyn sodium; iv) Theophylline; v) Epinephrine; vi) Cortisone.

Answer 330

1. Also known as cytotoxic hypersensitivity. 2. Involves antibody-mediated destruction. 3. Antigens are normally endogenous. 4. Primarily mediated by antibodies of the IgM and IgG classes and complement. 5. Reaction time is minute to hours. 6. Appearance-Lysis and necrosis. 7. Histology-antibody and complement. 8. Phagocytes and NK cells may also be involved (ADCC). 9. Mechanism-Ab directed against cell surface antigen mediates cell destruction via complement activation or ADCC. 10. Diagnostic tests- i) Detection of circulating antibodies against the tissues involved; ii) Presence of antibody and complement in the lesions by immunofluorescence 11. Treatment- anti-inflammatory agents and immunosuppressive drugs.

Answer 331

1. Also known as immune complex hypersensitivity. 2. Antigen is soluble. 3. Mediated by soluble immune complexes. 4. Reaction may take 3-10 hours to develop after antigen exposure. 5. Reactions develop when immune complexes activate complement. 6. Tissue damage is caused by platelets and neutrophils. 7. Appearance-erythema and edema, necrosis. 8. Histology-complement and neutrophils. 9. Mechanism- Antigen-antibody complexes deposited in various tissues induce complement activation and an ensuing inflammatory response. 10. Examples of diseases- Arthus reaction, Serum sickness, Systemic lupus erythematosus, Rheumatoid arthritis. 11. Diagnostic tests- i) Immunofluoresecent staining of tissue; ii) presence of immune complexes in sera by polyethylene glycol-mediated turbidity (nephelometry), binding of C1q and Raji cell test. 12. Treatment-Anti-inflammatory agents.

Answer 332

1. Also known as cell-mediated or delayed-type hypersensitivity reaction. 2. Involved in the pathogenesis of many auto-immune and infectious diseases. 3. Initiation time is 24-72 hours. 4. Appearance-erythema and induration. 5. Histology-mainly monocytes and few T cells. 6. Mechanism-Sensitized T cells release cytokines that activate macrophages and cytotoxic cells which mediate direst cellular damage. 7. Examples of diseases-tubercular lesions, contact dermatitis, celiac disease. 8. Diagnostic tests- i) delayed cutaneous reaction and patch test (in vivo); ii) mitogenic response, cytotoxicity assay and IL-2 production (in vitro tests). 9. Treatment- Corticosteroids and other immunosuppressive agents

Answer 333

By an increased susceptibility to infections. Primary or secondary These are generally chronic or recurrent infections without an obvious source, infections by organisms with normally low virulence, or infections of unusual severity. Primary immunodeficiencies are also associated with autoimmune diseases.

Answer 334

Primary immunodeficiency diseases: The immune deficiency itself is the cause of the disease. These are genetic disorders that usually - but not always - appear early in infancy or childhood. The databases show that while the effect at the protein level may be the same from patient to patient, the specific genetic defect can vary enormously. Secondary immunodeficiency diseases: The deficiency is the result of some other process. Examples include malnutrition, cancer, immunosuppressive drugs, or infection with HIV.

Answer 335

Secondary immunodeficiencies are far more common than primary immunodeficiencies. Secondary immunodeficiencies result from a variety of factors that can affect a host with an intrinsically normal immune system, including infectious agents, drugs, metabolic diseases, and environmental conditions. These deficiencies of immunity are clinically manifested by an increased frequency or unusual complications of common infections and occasionally by the occurrence of opportunistic infections

Answer 336

1. Worldwide, protein-calorie malnutrition is the most common cause of immunodeficiency. 2. Malnutrition can result from limited access to food sources and chronic diseases that induce cachexia, such as neoplastic diseases. 3. Diarrhea caused by infections and respiratory tract infections are common. T-cell production and function decrease in proportion to the severity of hypoproteinemia

Answer 337

a. Neonates have an increased susceptibility to common and opportunistic infections and sepsis compared with older children. b. There is an inverse association of infection susceptibility and the age of prematurity. a. Among the elderly, some subjects experience malignancies and an excessive number of infections caused by viruses and bacteria, reflecting a decrease in the immune defenses, particularly in the cellular compartment. b. Decreased delayed-type hypersensitivity skin reactions and decreased lymphocyte proliferative responses to mitogens can be demonstrated in this patient population.

Answer 338

1. Many disease processes originating from dysfunctional metabolic pathways significantly affect the cells involved in the immune response. 2. Diabetes mellitus and uremia resulting from kidney or liver disease are 2 common metabolic disorders with known deleterious effects on immunity.

Answer 339

1. Diseases caused by genetic defects might not primarily affect the immune system, but they can present with impaired immunity to infections resulting from metabolic and cellular dysfunction, such as poor expression of adhesion molecules or defects in the DNA repair machinery. 2. The molecular mechanisms leading to immunologic defects remain not well defined. Genetic syndromes are relatively rare, and usually only a subset of patients present with an immune defect of clinical severity that increases their risk of infections or malignancies. 3. The disease processes caused by chromosomal number abnormalities are the most common within the genetic disorders. As an example, patients with Down syndrome or trisomy of chromosome 21 present with increased incidence of infections, although they are usually not severe, including skin abscesses, periodontitis, and upper respiratory tract infections. T- and B-cell number and function are variably affected. Neutrophils isolated from patients with Down syndrome have shown defects in chemotaxis and phagocytosis in vitro.

Answer 340

The use of drugs to ameliorate undesirable immune responses is common in clinical practice as a consequence of the increasing prevalence of inflammatory conditions. These diseases include the categories of autoimmune disorders, allergic disorders, transplant rejection, and graft-versus-host disease (GvHD). The agents most available clinically and have in common their ability to inhibit lymphocyte proliferation and their lack of specificity for the immune response causing the particular illness of interest. Based on their structure and mechanism of action, most molecules with immunosuppressive activity can be grouped into corticosteroids, calcineurin inhibitors, and cytotoxic drugs. The adverse side effect of these drugs is that they tend to weaken the cellular immune response, rendering patients more susceptible to fungal and viral infections (acute, chronic, and reactivated).

Answer 341

The corticosteroids include both glucocorticoid and mineralocorticoid molecules. Only the glucocorticoids have significant anti-inflammatory activity. Glucocorticoids are well known for their variety of applications in both general and subspecialty medicine to reduce tissue damage caused by an excessive inflammatory response.

Answer 342

Calcineurin inhibitors bind cytoplasmic proteins from the immunophilin family and inhibit their interaction with calcineurin, which is essential for the activation of IL-2 transcription and T-cell function. The advantage of these drugs over corticosteroids and cytotoxic drugs is to spare macrophage and neutrophil functions, reducing the spectrum of susceptibilities to infections. However, these drugs cause respiratory tract and skin infections, usually of viral cause, to occur with increased frequency. The most common adverse effects of calcineurin inhibitors are hypertension and renal dysfunction; less common but more serious is the increased frequency of lymphoproliferative disorders and skin neoplasias. The first drug in this category was cyclosporine, which has been extensively used to prevent organ transplant rejection, GvHD, and corticosteroid-resistant autoimmune disorders.

Answer 343

Cytotoxic agents were conceived to control neoplastic cell growth and ablate the bone marrow for transplantation. They have progressively found their niche in the immunosuppressive drugs category because of the selectivity conferred by the proliferative nature of the immune response, and their application has extended to autoimmune and inflammatory disorders, including GvHD and the prevention of graft rejection.

Answer 344

For example, antibodies that interfere with TNF signaling can lead to increased reactivation of TB

Answer 345

1 Surgery and trauma cause disruption of epithelial barriers and cell destruction that triggers an inflammatory response to promote healing and local microbicidal activity. 2 Removal of spleen

Answer 346

1 There is increased awareness of potential adverse effects caused by chronic exposure to inhospitable environmental conditions, such as extreme cold or high altitude. 2 It has been recommended to avoid exposure to sunlight because of increased risk of malignancies; however, beneficial effects of sunlight have also been observed, particularly in patients with skin inflammatory conditions, such as psoriasis 3 The biologic effect of sunlight in inflammation is mediated by UV light, which induces T-cell apoptosis, nonspecific release of tolerogenic cytokines from antigen-presenting cells in the epidermis, and differentiation of regulatory T cells; hence UV light is used in the treatment of eczema and the skin manifestations of autoimmune disorders. 4 The immunosuppressive effect of ionizing radiation affects all blood cell lineages by depleting the bone marrow and inducing cytopenias, whereas the humoral response and phagocytosis are considered radioresistant. However, continuous exposure to radiation eventually weakens all immune functions.

Answer 347

1 Transient periods of immunosuppression have been associated with viral infections since the 1900s, when it was observed that tuberculin skin test results became negative in patients with measles during the acute phase of the infection. 2 Infections with measles virus, CMV, and influenza virus can induce lymphopenia and also T-cell anergy; however, these are transient and usually less severe than the immunodeficiency seen in AIDS. 3 One additional mechanism of immune compromise is infection of the bone marrow by viral and bacterial organisms producing neutropenia or pancytopenia, particularly in immunocompromised hosts

Answer 348

1. Bruton's agammaglobulinemia (X-linked agammaglobulinemia or XLA). This disease was described in 1952 and was the first immunodeficiency disease to be recognized. The defect has been mapped to a gene on the long arm of the X chromosome, which encodes for a cytoplasmic tyrosine kinase (also known as B cell tyrosine kinase or Bruton's tyrosine kinase or BTK). The gene is normally expressed at all stages of B cell differentiation and is required for transducing signals that are required for maturation. Pre-B cells are present in the bone marrow but there is no further B cell differentiation. There is an absence of all classes of Igs. 2. X-linked hyper IgM syndrome. Elevated levels of IgM. Deficiency of other immunoglobulins. Defect is in the gene that encodes CD40L. 3. Selective immunoglobulin deficiencies a. IgA – most common immunodeficiency disease. b. IgM c. IgG 4. Common variable hypogammaglobulinemia (CVID). This is a heterogeneous group of deficiencies that affect both males and females at any age including adults. The diagnosis is a descriptive one and is made once all other syndromes have been excluded. B cell counts can be normal but the cells fail to mature into antibody-secreting cells. T cell numbers are normal.

Answer 349

Complement abnormalities are generally associated with increased susceptibility to infections and increased incidence of autoimmune diseases. The most common deficiency associated with the complement system is C1 inhibitor deficiency known as hereditary angioneurmtic edema (HANE). Patients with this deficiency have recurrent swelling of various parts of the body. When the swelling involves the upper airway, the patients may die from obstruction. If it involves the intestine the patients have abdominal pains and cramps. This deficiency may occur later in life due to the development of autoantibodies.

Answer 350

1. Chronic granulomatous disease (CGD) - defective production of NADPH oxidase. Superoxide anions and hydrogen peroxide cannot be formed and ingested microorganisms cannot be killed. The diagnosis is made by the inability of phagocytes to reduce nitroblue tetrazolium (NBT) dye after a stimulus. IFN-γ therapy is now commonly used for the treatment of X-linked CGD. 2. Leukocyte adhesion deficiencies a. Leukocyte adhesion deficiency type 1 (LAD-1) – defective production of CD18, the ß2 integrin subunit. It is normally found on neutrophils, monocytes, and lymphocytes. These cells cannot adhere to vascular endothelial cells, which prevents extravasation. Its absence also prevents CTLs and NK cells from adhering to target cells as well as preventing T and B cells from forming conjugates. b. Leukocyte adhesion deficiency type 2 (LAD-2) – a generalized defect in fucose metabolism due to defective production of the leukocyte ligands that are required for binding to endothelial E- and P-selectins. This leads to a failure of leukocyte migration. 3. Chediak-Higashi syndrome – autosomal recessive disease that results from the inability of phagocytes to lyse bacteria. Defect is due to a mutation in a protein (LYST) involved in the regulation of intracellular trafficking.

Answer 351

Severe Combined Immunodeficiency Disease(s) (SCID) – a genetically heterogeneous group of diseases in which the development and/or function of both T and B cells is profoundly impaired. The lesions may affect T cells primarily with B cell dysfunction resulting from the absence of necessary stimulatory interactions and cytokine. In some cases both B and T cells may directly be affected. SCIDs usually present in infancy with variable combinations of the following: failure to thrive, oral and/or cutaneous candidiasis, chronic diarrhea, and opportunistic infections as well as other types of infections.

Answer 352

X-linked recessive form ("Boy in the Bubble"). Few, if any, T cells in the circulation characterize this form. There are usually normal B cell levels but with severe hypogammglobulinemia since the B cells do not receive necessary help for antibody formation. Most X-linked SCID cases are due to mutations encoding the common gamma (γ2)-chain – the signal-transducing element – shared by the receptors for IL-2, IL-4, IL-7, IL-9, and IL-15. In this case, IL-7, the cytokine that stimulates the growth of immature thymocytes, cannot exert its normal effect.

Answer 353

- This type results from enzyme deficiencies. There is a lack of adenosine deaminase (ADA) or purine nucleoside phosphorylase (PNP). These patients have few T cells or B cells and exhibit severe hypogammaglobulinemia. These enzyme deficiencies lead to a buildup of toxic products within lymphocytes. - This type can lead to a defect in antigen receptor gene recombination and result in the absence of mature B and T cells.

Answer 354

a. ZAP-70 deficiency b. Bare Lymphocyte Syndrome i. Defective MHC class II expression patients express little or no HLA-DP, HLA-DQ, or HLA- DR on APCs and fail to express MHC class II in response to IFN-γ This leads to deficient DTH responses and antibody responses to T-dependent antigens. ii. Defective MHC class I expression – characterized by decreased numbers of CD8+ T cells. In some cases there is a defect in the gene that encodes TAP that delivers peptides to the ER so that they can bind to MHC class I for presentation. These patients generally have respiratory tract bacterial infections rather than viral infections that would have been expected.

Answer 355

1. Wiskott-Aldrich syndrome (WAS) - characterized by eczema, thrombocytopenia, and bacterial infections. The defective gene responsible for the disease encodes a cytosolic protein that plays important roles in signal transduction and the regulation of the cytokskeleton in lymphocytes. 2. Ataxia telangiectasia - characterized by cerebellar ataxia, oculocutaneous telangiectasias, a variety of immune deficits, radiation sensitivity, and a high incidence of lymphoma. γδT cells are about 50% of the total number of peripheral lymphocytes whereas in normal individuals they are about 1-5%.

Answer 356

DiGeorge anomaly/DiGeorge syndrome (congenital thymic aplasia). 1st described in 1965. The disease is due to a deletion in the chromosome 22q11.2. Patients are susceptible to mycobacterial, viral, and fungal infections. It is associated with symptoms apparent immediately following birth including: 1. Absence of the thymus due to failure of the development of the 3rd and 4th pharyngeal pouches during gestation 2. Hypoparathyroidism – abnormal calcium homeostasis and muscle twitching 3. Cardiovascular anomalies 4. Abnormal faces – defects in the ears, mouth, jaw, and eyes

Answer 357

A. Examination of peripheral blood smear or flow cytometric patterns B. Humoral immunity 1. Measurement of serum Ig levels 2. Response to immunization C. Cell-mediated immunity – good screening tests are lacking 1. T cell count – however, lymphopenia is not always present in patients with functional T cell defects 2. Flow cytometric analysis of cell surface markers 3. DTH skin reactions – require prior exposure, cannot be used in children under the age of 12 months D. Complement – total serum complement can be measured – assays can measure levels of each of the individual complement components E. Phagocytic cell function – functional assays (chemotaxis, phagocytosis, bactericidal activity)

Answer 358

``` A. Gamma globulin B. Bone marrow transplantation C. Antibiotics D. Cytokines E. Gene therapy ```