Test 1

Question 1

Why was Jenner’s vaccine superior to previous methods for conferring resistance to
smallpox?

Answer 1

The previous method that had been used to confer smallpox immunity used material from lesions of smallpox victims. While it conferred immunity, it was still possible to contract and even die from the disease. By using cowpox to confer immunity to smallpox significantly reduced the risk of acquiring the potentially lethal disease.

Question 2

Describe how Pasteur developed & tested his anthrax vaccine and why it was important.

Answer 2

Louis Pasteur discovered how to attenuate a pathogen and administer that attenuated pathogen to act as a vaccine. He developed the attenuated vaccine against anthrax and then set up a controlled experiment in sheep. This marks the beginning of the discipline of immunology given that we now have so many ways to develop effective vaccines.

Question 3

What is herd immunity and why is it important clinically?

Answer 3

Herd immunity occurs when enough members of a population have protective immunity to a pathogen, either through vaccination or prior infection. These individuals act as a buffer to spread of disease and help protect those that have weak immune systems or no immunity (i.e. allergic to components of the vaccine, immunocompromised).

Question 4

If vaccines are so successful, why have they been controversial for some people?

Answer 4

Because of a claim perpetuated in the Lancet that vaccines cause autism, this myth became
“fact” for many people, despite the article’s retraction. The timing of the peak number of
vaccinations at 2 years and the development of developmental disorders do coincide, but there
is no scientific literature that suggests that vaccines cause autism. Past controversies with
thimerosal use in vaccines also contribute to confusion for parents.

Question 5

Give four examples of mild and severe consequences of immune dysfunction?

Answer 5

Mild – sneezing, hives, skin rash
Severe – anaphylactic shock, susceptibility to infection (immunosuppression), chronic debilitation (autoimmunity)

Question 6

List three differences between innate and adaptive immunity?

Answer 6

1) Innate response time is fast, whereas the adaptive response is slower.
2) **Innate response specify is limited and fast, Adaptive is diverse and improve over time.
3) Innate response is the same whereas adaptive response is more rapid & effective over time

Question 7

How is the recognition of antigen different between pattern recognition receptors and B and
T cell receptors?

Answer 7

PRR are germline-encoded receptors and recognize general pathogen associated molecular
patterns (PAMPS). This type of recognition is the hallmark of innate immunity.

B and T cell receptors are a result of DNA rearrangement and are much more diverse and respond to specific pathogens. This type of response is the hallmark of adaptive immunity.

Question 8

How is clonal selection important for the adaptive immune response?

Answer 8

Adaptive immune receptors are randomly made using DNA rearrangement and then pathogen
specific cells are selected for based on their antigen binding from a diverse group of receptors.
This theoretically provides the ability to recognize and respond to any antigen.

The idea is that each cell has one unique receptor; this cell then proliferates in response to antigen, providing a tremendous force of immune cells specific to that antigen.

Question 9

What are memory cells and why are they important for protection from infection?

Answer 9

Upon exposure to antigen, B and T cells clonally expand and memory cells are formed. When
exposed to the same pathogen, these memory cells respond faster and more robustly to remove
the infectious agent more efficiently.

Question 10

What is the hygiene hypothesis and how does it relate to hypersensitivity reactions?

Answer 10

The hygiene hypothesis predicts that a lack of early childhood exposure to infectious agents and parasites increases susceptibility to allergic diseases by modulating immune system
development.

Increases in hypersensitivity reactions can be explained by alterations in the development of immune cells that were not exposed to many pathogens in development.

Question 11

What is the difference between a pluripotent stem cell and a hematopoietic stem cell?

Answer 11

Both pluripotent and hematopoietic stem cells are able to “self-renew” and able to differentiate into different cell types.

A pluripotent stem cell possesses the capability to develop into any kind of specialized cell, regardless of cell tissue or type.

A hematopoietic stem cell, however, can only generate cells that arise from the blood and mature into specialized blood cells such as macrophages, lymphocytes, red blood cells, dendritic cells, and granulocytes.

Question 12

List the cells that differentiate from the myeloid progenitors

Answer 12

Eosinophils
Neutrophils
Basophils
Erythrocytes
Megakaryocytes (platelets)
Monocytes
Macrophages
Dendritic cells
Mast cell

Question 13

List the cells that differentiate from the lymphoid progenitors

Answer 13

T lymphocyte
B lymphocyte
Natural killer cell
Dendritic cell

Question 14

What are the characteristics of granulocytes?

Answer 14

Granulocytes consist of neutrophils, basophils, eosinophils, and mast cells.

These cells are part of the innate immune system and are the immune system’s first defense against foreign invaders. Granulocytes have multi-lobed nuclei—lymphocytes’ nuclei are round.
They have granules in their cytoplasm (hence the term granulocyte) that are released when
an infection is detected. Of the granulocytes, neutrophils have the largest number of cells,
comprising 50-70% of leukocytes.

Question 15

What is the difference between a monocyte and a macrophage and a dendritic cell?

Answer 15

These cells are derived from myeloid progenitors and all are phagocytic cells that present antigens to other immune cells such as T lymphocytes.

Monocytes are the predecessors of macrophages and dendritic cells. Monocytes can either be inflammatory or patrolling, meaning they can enter tissue or crawl along blood vessels respectively.

The monocytes that migrate into the tissue become macrophages and typically become tissue-specific, such as osteoclasts and microglial cells. Macrophages use pseudopodia to phagocytize and present antigen, becoming activated and more effective when interacting with specific antibodies or T helper cells.

Dendritic cells appear to arise from both myeloid and lymphoid progenitors. In processing antigen, dendritic cells can phagocytize or internalize it via receptor-mediated endocytosis or pinocytosis using elongated processes, then presents it to naïve T cells, activating them.

Question 16

What are the three main types of lymphocytes and what is the purpose of the term “CD” in immunology?

Answer 16

The three main types of lymphocytes are B cells, T cells, and ** natural killer cells**.

Under a microscope, these cells look very similar; because of this, the surface markers on each of
these cells must be used to identify each type. These surface proteins are the “CD” or cluster of differentiation. By using the CD nomenclature, each of these surface proteins has a specific name associated with the CD nomenclature, such as CD4, which helps distinguish different cell subtypes.

Question 17

List two primary and secondary lymphoid organs and summarize their functions in the
immune response.

Answer 17

The two primary lymphoid organs include the bone marrow and the thymus. Both of these
organs are where myeloid and lymphoid cells differentiate or mature; the bone marrow is
where the hematopoietic stem cells reside and B cells mature, while T cells undergo
selection and maturation in the thymus.

The bone marrow functions to develop and replenish blood cells by providing the environment necessary for those cells to grow and differentiate.

The thymus provides specific microenvironments which allow for immature T cells (thymocytes) to develop and mature into a specific type of T cell. In the thymus, the T cells generate unique receptors that are then selected based on whether or not they react to self-peptide.

Two secondary lymphoid organs include lymph nodes and the spleen.

Question 18

Summarize how positive and negative selection of T cells in the thymus works.

Answer 18

Immature T cells enter the thymus so they can develop their T cell receptors (TCRs). When
they enter the thymus, the TCRs interact with thymic stromal cells that have self MHC peptides presented on their surface.

The T cells whose TCRs bind really tightly to those self MHC peptides will undergo negative selection and will be forced to die.

If the TCRs don’t bind too tightly or only have an intermediate affinity to the self MHC peptides (in other words, binding has to be just right), the T cell will undergo positive selection and be allowed to live and develop into a mature T cell.

Question 19

Describe the three functional regions in a lymph node.

Answer 19

The lymph node has three distinct regions that include the cortex, the paracortex, and the
medulla.

The cortex is the outermost layer and consists of follicles full of B cells, macrophages, and dendritic cells. The cortex is connected to the body via afferent lymphatic vessels, which bring in antigen traveling as a particulate from an infected tissue
or with an antigen-presenting cell.

The paracortex is deep to the cortex and is where T cells and dendritic cells typically reside. The naïve T cells travel along fibroblast reticular cells in the paracortex to guide their movements as they sample the MHC peptides presented by the dendritic cells.

The medulla is the innermost layer and is where lymphocytes exit the lymph node via efferent lymphatic vessels, so not very many cells are found here in comparison to the cortex and paracortex.

Question 20

Describe the three functional regions in a spleen.

Answer 20

While the lymph nodes specialize in providing interactions between antigen and lymphocytes, the spleen’s responsibility is to filter blood and trap antigens in the blood.

The three regions of the spleen include the red pulp, white pulp, and marginal zone. There is an outermost layer called the capsule which consists of trabeculae that extend out to
provide support for the spleen.

The red pulp consists of sinusoids where red blood cells, macrophages, and lymphocytes reside and is the place for old or defective red blood cells to
be destroyed.

The marginal zone separates the red and white pulp and is where macrophages and B cells make the first defense against blood-borne pathogens.

The white pulp consists of T cells and B cell follicles, where germinal centers generate more B cells with increased receptor affinity.

Question 21

What is a M cell and why is it important in mucosa associated lymphoid tissue?

Answer 21

An M cell is a specialized epithelial cell residing in the digestive tract’s mucosa-associated
lymphoid tissue that transports antigen across the epithelium.

They accomplish this by using a deep pocket that contains B cells, T cells, macrophages and dendritic cells. Antigens are trapped in vesicles, then transported to this pocket membrane; the cells residing in the pocket process the antigen then travel below the epithelium to activate B cells. These B
cells differentiate into plasma cells and produce IgA antibodies that target the mucosal infection. This provides an effective tool in protecting the digestive system.

Question 22

What is different about jawless and fish with jaws regarding their immune systems?

Answer 22

Fish with jaws have more structured primary and secondary lymph structures, such as the thymus and spleen.

Jawless fish, on the other hand, do not have such structural organization in their immune system.

It was thought that the jawless fish didn’t have
thymic tissue, but they have found that there are distinct populations of cells similar to T
and B cells and they might have thymic tissue near their gills.

Question 23

Describe what the terms cytokine, interleukin, and chemokine mean.

Answer 23

Cytokines refer to molecules that communicate among immune cells. They are generally
soluble molecules which bind to receptors on target cells and cause changes in the expression
of adhesion molecules and chemokine receptors, affect enzymatic or transcriptional factor
activity, or instruct a cell when to live or when to die.

Interleukins are derived from the Latin
words inter (between) and leukocytes (white blood cells) and is a term typically used in naming different cytokines.

A chemokine refers to a specific kind of cytokine that serve to mobilize immune cells from one organ or tissue to another. These molecules attract cells with the right chemokine receptors to where chemokine concentration is the highest.

Cytokines can be further classified by the distance they are secreted: endocrine (pass through the
bloodstream), paracrine (very short distances between cells or tissues), or autocrine (cell
signals itself).

Question 24

Describe pleiotropy, redundancy, synergy, and antagonism.

Answer 24

Pleiotropy means that one cytokine has multiple target effector cells that it affects.

Redundancy implies that different cytokines cause or do the same function (not summation).

Synergy means that two or more cytokines work together to produce a specific effect (summation).

Antagonism is where one cytokine blocks or antagonizes the actions of another.

Question 25

Distinguish between a hormone, a cytokine, a chemokine, and a growth factor. What
functional attributes do they share, and what properties can be used to discriminate among
them?

Answer 25

All four are chemical signaling molecules, which are often, but not always, proteins. They
deliver a message to a second cell, causing physiological changes in the second, target cell,
which must bear receptors that are specific for the signaling molecule.

Growth factors are often secreted constitutively, whereas cytokines and hormones are secreted in response to particular stimuli and their secretion is short-lived.

Most hormones are produced by specialized glands and their action affects a small number of target cells.

In contrast, cytokines are often produced by, and bind to, a variety of cells.

Chemokines are specialized cytokines that provide chemotactic signals which in*duce target cells to move in a particular direction toward the cell secreting the chemokine. *

Question 26

What is the role of IL-1 family cytokines in infection?

Answer 26

The** IL-1 family cytokines** are typically secreted very early in the immune response by dendritic cells and monocytes/macrophages.

Secretion of these kinds of cytokines is usually due to recognition of viral, parasitic, or bacterial antigens.

Their primary role is creating a proinflammatory response by increasing the capillary permeability and leukocyte migration. They also produce systemic effects, such as a fever, and signal the liver to produce acute phase proteins like the Type I Interferons to further protect the body, such as destroy viral RNA.

Question 27

How do activated T cells change so they are more sensitive to IL-2?

Answer 27

When T cells are activated, they change the conformation of their IL-2 receptors. 3 different
types of IL-2 receptors exist: low-affinity, intermediate-affinity, and high-affinity.

Resting T cells typically express intermediate-affinity IL-2 receptors, which consists of the γ and β
subunits.

Activated T cells express both the low-affinity receptor, which consists only of the α
subunit, and the high-affinity receptor, which consists of all three subunits. The low-affinity
receptor can only bind to IL-2, the intermediate-affinity receptor can bind and create signal
transduction, and the high-affinity receptor is responsible for most physiologically relevant
IL-2 signaling because all three subunits provide additional contacts with the IL-2 ligand.

Why there are both low and high affinity receptors on activated T cells is not entirely understood but is believed that the low affinity receptor binds ligand to be passed to the high-affinity receptor or it may reduce cytoplasmic levels of IL-2 so only cells with the high-affinity receptor will be activated.

Question 28

What are examples of Type I, Type II, and Type III interferons?

Answer 28

Type I interferons are typically IFN-α and IFN-β, are secreted by activated macrophages, dendritic cells, and virus-infected cells, and are used to treat a variety of diseases, i.e. hepatitis.

Type II interferons are typically IFN-γ, are produced by activated T and NK cells, are powerful modulators of the adaptive immune system by biasing T cells to become TH1 and activating macrophages.

Type III interferons are the most recently discovered IFN-λ which upregulate the expression of genes controlling viral replication and host cell proliferation.

Question 29

Is the TNF family of cytokines soluble, membrane bound, or both?

Answer 29

This family of cytokines can be both soluble and membrane bound.

Oftentimes, they are firmly anchored to the cell membrane, consisting of a long extracellular C-terminal region, a Type 2 transmembrane protein connected to a short intracellular N-terminal region. The extracellular region can be cleaved to form a soluble cytokine or the membrane bound form
can exist as a soluble form.

Question 30

Describe one mechanism by which Type I interferons “interfere” with the production of
new viral particles?

Answer 30

When a Type I interferon binds to its receptor on a virally infected cell, the interferon signal results in the activation of a ribonuclease that breaks down cytoplasmic RNA.

It is particularly effective against double-stranded RNA.

Question 31

How do chemokines affect chemotaxis?

Answer 31

Chemokines primarily direct chemotaxis (the speed and direction of cell movement) by using
concentration gradients of signaling molecules. This results in a cascade of cellular signaling which activates Rho (which is responsible for cell movement). This causes actin protrusions to form, which cause actin/myosin contraction, which help move the cell along. This all happens within 30 seconds of chemokine binding.

Question 32

What type of signaling are chemokines dependent upon?

Answer 32

Chemokines rely on G protein signaling in order to cause chemotaxis.

These G proteins are associated with GTP-binding proteins that initiate a signaling cascade within the cell. This cascade activates downstream pathways such as the Ras/MAP kinase pathway and the Rho pathway which promotes cell migration.

Question 33

How do epithelial barriers prevent pathogen entry into the body’s interior and how are microbes that breach the skin barrier recognized?

Answer 33

Epithelial barriers primarily serve as physical barriers to pathogens by blocking their entrance into the body. The skin consists of two layers, the epidermis and dermis; the epidermis has several layers of packed keratinocytes while the dermis consists of connective tissue and blood vessels, hair follicles, sweat glands, sebaceous glands, and myeloid leukocytes.

These myeloid leukocytes (in dermis) stand as sentinels in case pathogens pass through this physical barrier. Mucosal surfaces are protected by cells joined by tight junctions so pathogens can’t squeeze past. These physical barriers also serve as a chemical barrier by producing antimicrobial substances that have a low pH. Secretions such as saliva and urine wash away and kill microbes; mucus specifically entraps foreign microorganisms and cilia move them out from the body.

The epithelial cells secrete many proteins that have antimicrobial properties; many are enzymes that bind to and kill/inhibit microbes. Lysozyme, found in saliva, tears, and respiratory tract fluids cleaves peptidoglycan. Antimicrobial peptides also contribute to innate immunity. They are generally less than 100 aa, cysteine-rich, positively charged, and
amphipathic; these abilities allow them to interact with acidic phospholipids to disrupt microbe membranes, then inhibit synthesis of DNA, RNA, and proteins, thereby killing the cell. The main peptides are α- and β-defensins and cathelicidin, as well as surfactants SP-A and SP-D. These microbes are recognized by PAMPs, pathogen-associated molecular patterns, which are on their surface; PAMPs are detected by PRRs, pattern recognition receptors. When PAMPs are recognized by PRRs on phagocytic cells, this induces phagocytosis of the microbe.

Question 34

List the cells that play a key role in the phagocytosis and describe how a microbe is destroyed after it has been phagocytized.

Answer 34

Macrophages, monocytes, neutrophils, and dendritic cells play the primary role of carrying
out phagocytosis.

When their PRRs recognize PAMPs, the cells extend their plasma membrane and engulf them using actin polymerization, thereby internalizing them in phagosomes. Once internalized, these phagosomes fuse with lysosomes which kill and degrade the microbe using antimicrobial proteins and peptides, low pH, hydrolytic enzymes and reactive oxygen and
nitrogen species.

Question 35

How do opsonins function to improve innate immunity?

Answer 35

Opsonins are soluble phagocytosis-enhancing proteins that bind to conserved, repeating
components of microbes.

These opsonins bind to the microbial surface of pathogens and are recognized by membrane opsonin receptors on phagocytes, thereby inducing phagocytosis.

Examples include SP-A and SP-D, L-ficolin, and C1q (a complement component), which are all
bound by the CD91 opsonin receptor. C-reactive protein (CRP) and IgA/IgG antibodies also
function as opsonins and are bound by the Fc receptor

Question 36

What are the cell wall differences between gram negative and gram positive bacteria?

Answer 36

Gram-negative bacterial cell walls consist of an outer and inner membrane separated by peptidoglycan; they express lipopolysaccharides (endotoxins) on their surface.

Gram-positive bacteria have only one membrane with a layer of peptidoglycan on top; glycolipids and glycoproteins are expressed on the surface.

Question 37

Describe how Toll like receptors recognize and respond to specific PAMPs?

Answer 37

TLRs are membrane-spanning proteins that have leucine-rich repeats in their extracellular
domain, which creates a horseshoe-shaped ligand-binding domain.

When they bind PAMPs on the surface, the TLRs are induced to dimerize, forming a characteristic m shape. They also recognize DAMPs, which are self-peptides such as heat shock and chromatin proteins that indicate a cell is dying or dead. When TLRs bind to their specific PAMPs, **dimerization occurs and activates either the MyD88 or TRIF pathway. **

The MyD88 pathway activates NF-kB and
AP-1; the* TRIF pathway also does this but also activates IRF3 which dimerizes with IRF7 to
induce transcription of Type I interferons*.

Question 38

Why is cellular location of TLRs important in terms of their function?

Answer 38

TLRs exist on both the plasma membrane and membranes of lysosomes and endosomes.

TLRs on the surface are stimulated by microbial surface proteins; in contrast, TLRs on the inside of the cell must be stimulated by endocytosed pathogens’ nucleic acids .

TLR-4 is unique in that it can move from the plasma membrane to the endosomal membrane after binding its PAMP. This is important because both pathways activate different signaling pathways depending on the TLRs location. TLRs on the surface primarily activate the MAP kinase pathway to produce AP-1 and the NF-kB pathway to produce NF-kB.

Endosomal TLRs primarily activate IRFs, which induce transcription of the genes encoding IFN-a and IFN-B.

Question 39

What is the difference between CLRs, RLRs and NLRs?

Answer 39

CLRs (C-type lectin receptors) are plasma membrane receptors that generally recognize
carbohydrate components of fungi, mycobacteria, viruses, parasites, and some allergens. Some CLRs function as phagocytic receptors. Their signaling is similar to the MyD88-dependent signaling which activates NF-kB and AP-1, which induce expression of inflammatory cytokines and IFN-β.

RLRs are soluble PRRs that reside in the cytosol and sense viral infections by recognizing viral RNAs. When they bind to ds-RNA, virus-specific sequence motifs, or a 5’ triphosphate modification, they activate a downstream cascade which activates IRFs 3 and 7 and the NF-kB pathway which expresses IFN-α and β.

NLRs are cytosolic proteins that are activated by intracellular PAMPs and DAMPs; NOD1 and NOD2 specifically are activated by peptidoglycans. They trigger inflammation.

Question 40

What is an inflammasome and why is it important?

Answer 40

An inflammasome is formed by a complex of NLRs which assemble with other proteins that
activate proteases necessary for cleaving inactive large precursor cytokines, generating the
mature proinflammatory cytokines.

This is very important so that caspase-1 can cleave pro-IL-1 and pro-IL-18; without it, no mature cytokines would be produced and released.

Question 41

What are the 4 steps that initiate a local inflammatory response?

Answer 41

**1. **Tissue damage and bacteria (PAMPs and DAMPs) activate resident sentinel cells to release chemoattractants, proinflammatory cytokines TNF-α, IL-1, and IL-6, and vasoactive factors that trigger a local increase in blood flow and capillary permeability.

**2. **Permeable capillaries allow an influx of fluid (exudate) and cells.

3. Neutrophils and other phagocytes migrate to site of inflammation.

4. Phagocytes and antibacterial substances destroy bacteria, clear invading pathogens, dead cells, and damaged tissue.

Question 42

Why are dendritic cells so important for bridging innate and adaptive immunity?

Answer 42

Dendritic cells are so important in bridging innate and adaptive immunity in that they carry
microbial components from the epithelial tissues (the innate system) and present them to
lymphoid tissues where T and B cells (the adaptive immune system) can recognize it.

Binding of PAMPs also activates the dendritic cell to mature so that it expresses higher levels of MHC
class II proteins and costimulatory membrane proteins so Th cells can be activated. They can
also cross-present to Tc cells by presenting endosomal/lysosomal degradation products on
MHC class I proteins.

Question 43

What is the complement system?

Answer 43

The complement system consists of a** complex group of more than 30 glycoproteins that reside
in the serum which cooperates with both the innate and adaptive immune system to eliminate
blood and tissue pathogens.**

They are synthesized in the liver and make up approximately 15% of the globulin protein fraction in plasma. They can be classified into seven functional categories:

Initiators initiate complement cascades by binding to soluble or membrane-bound particles, then undergo conformational changes that change their activity.

Enzymatic mediators are proteolytic enzymes that cleave or activate members of the complement cascade.

Opsonins bind to pathogens and enhance
phagocytosis.

Inflammatory mediators enhance the blood supply to the area where they are released by increasing the diameter of the blood vessel. In excess, they cause anaphylaxis.

Membrane attack proteins form the membrane attack complex (MAC) which punches holes into
microorganisms.

Complement receptor proteins simply bind complement and signal specific pathways such as phagocytosis, degranulation, and inflammation.

Regulatory complement components protect cells from unintended complement-mediated lysis.

Question 44

How are the three main pathways of complement activation similar?

Answer 44

All three pathways converge to generate an enzyme complex capable of cleaving the C3 molecules into C3a and C3b.

The enzymes that cleave C3a and C3b are referred to as C3 convertases.

C3a is a small anaphylatoxin, whereas C3b is pivotal for opsonization. C3b and the C3 convertases then form the C5 convertases, which cleave C5 into C5a, which is an inflammatory mediator, and C5b which is the initiating factor of the MAC.

Question 45

How are the three main pathways of complement activation different?

Answer 45

The classical pathway is initiated by antibody binding, specifically by IgM or IgG antibodies, so it
is considered to be part of the adaptive immune system.

The lectin pathway is initiated by lectins, which are proteins that recognize specific carbohydrate components specific to microbial cell surfaces. Mannose-binding lectin is a PRR which also recognizes sugars, so the lectin pathway is considered to be part of the* innate immune system*.

The alternative pathway, which is the most ancestral of the complement pathways, is independent of antibody-antigen interactions, so it is also classified as part of the innate immune system. The alternative pathway is initiated after spontaneous hydrolysis of proteins or association with specific blood proteins or proteases.

Question 46

What are the main steps of the classical complement pathway?

Answer 46

1. The C1q component of complement binds to at least 2 CH2 domain of the antigen-bound antibody molecule, activating C1r which cleaves and activates the two C1s molecules.
2. C1s cleaves C4 into C4b. C4b binds to the membrane close to C1, then binds C2 which is
   cleaved by C1s. Cleavage of C2 forms the C2 convertase.
3. C3 convertases hydrolyze C3 into C3a and C3b. C3a is a small anaphylatoxin, while C3b binds to the microbial surface, opsonizing it. C3b molecules also combine with C3 convertase to form C5 convertase.
4. The C5 convertase binds and cleaves C5 into C5a which is an inflammatory mediator and C5b which is the initiating factor of the MAC.

Question 47

What is the membrane attack complex and how is it formed?

Answer 47

The membrane attack complex consists of molecules C5b, C6-C9, which punches holes in the cell membrane of pathogens.

C5b provides the binding site for the rest of the components of the MAC, is stabilized by C6, then C7 binds. Once C6 and C7 bind, the complex undergoes a conformational change which exposes hydrophobic regions of C7, making it capable of inserting itself into the cell membrane. This allows C8 and C9 to bind and insert into the membrane, creating the MAC.

Question 48

List the three main functions of complement?

Answer 48

1. Innate defense against infection.
a. Lysis of bacterial and cell membranes
b. Opsonization
c. Induction of inflammation and chemotaxis by anaphylatoxins

2. Interface between innate and adaptive immunity.
a. Augmentation of antibody responses
b. Enhancement of immunologic memory
c. Enhancement of antigen presentation

3. Complement in the contraction phase of the immune response.
a. Clearance of immune complexes from tissues
b. Clearance of apoptotic cells
c. Induction of regulatory T cells

Question 49

What is the role of CD21 in altering B cell activation and why is this important?

Answer 49

CD21 is a co-receptor found on the surface of B cells which enhances avidity and B cell activation sensitivity.

Binding of both CD21 and the BCR reduces the antigen concentration necessary for B cell activation by up to one hundred fold. CD21 also plays an important role in bacteria DNA recognition and B cell tolerance.

Question 50

How are red blood cells critical in clearance of immune complexes?

Answer 50

RBCs have complement receptors expressed on their cell membranes which bind with high affinity to immune complexes. RBCs can deliver the C3b-coated immune complexes to the liver and spleen where the immune complexes can be phagocytized. This clears immune complexes from the blood.

If this didn’t happen, immune complexes left in the serum would be deposited in the tissues, activating complement and induce pathological inflammation.

Question 51

Why would deficiencies in complement increase your risk for developing Lupus?

Answer 51

There are two hypotheses for this:

The clearance hypothesis states deficiencies in complement result in apoptotic cells and immune complexes staying in circulation longer and initiating
autoimmunity.

The tolerance hypothesis proposes that the decreased sensitivity of developing B cells results in auto-reactive B cells surviving selection and becoming hyperactive.

Question 52

What are the four main microbial complement evasion strategies?

Answer 52

1. Interference with antibody-complement interaction
a. Synthesis of proteins and glycoproteins that bind the Fc regions of the antibody or internalization of protein-antibody complexes.

2. Binding and inactivation of complement proteins.

3. Protease-mediated destruction of complement proteins.

4. Microbial mimicry of complement regulatory proteins.

Question 53

Describe the experiment Tonegawa performed to determine that DNA recombination occurs in somatic cell immunoglobulin genes.

Answer 53

Isolated germline DNA from embryonic liver and from antibody producing tumor cells.

Cut the DNA with a restriction endonuclease.

Made radioactive light chain mRNA probes (full length and 3’ end).

Cut gel into slices, added probes, washed, read, and compared DNA samples.

Showed that antibody genes were different sizes in B cells compared to embryonic cells.

Question 54

What genes combine to form the antibody light and heavy chains respectively?

Answer 54

Antibody light chains are encoded in two segments (V and J), whereas antibody heavy chains are encoded in three segments (V, D, and J).

Question 55

What are the names of the two light chains and how abundant are they in mice and man?

Answer 55

The light chains are called kappa or lambda, with the kappa present 95% of the time in the mouse (lambda on 5%)** whereas in humans there is a 60%:40% distribution, respectively**.

Question 56

What is the recombination signal sequence (RSS) and what does it do?

Answer 56

Recombination signal sequences flank each antibody gene segment. Each has a conserved
nonamer and heptamer sequence.

In between the nonamer/heptamer is a 12 or 23 bp spacer sequence (allows for 1 or 2 turns of DNA). Recombination is directed by signal sequences.

Question 57

What is the role of Rag1 and Rag2?

Answer 57

Gene segments are joined by the RAG1/2 recombinase.

RAG = recombination activating gene.

Both proteins are needed for recombination. RAG1 is more important—it forms a complex with RSSs stabilized by binding RAG2.

Question 58

What is the role of artemis and why is it important?

Answer 58

After RAG1/2 have bound V and J gene segments, nicked the 5’ end, and the ends have ligated,artemis opens the hairpin loop which can generate 5’ or 3’ ends that can increase P nucleotide addition and antibody diversity.

Artemis is essential for V(D)J recombination to occur.

Question 59

List the 5 mechanisms of antibody diversity generation.

Answer 59

1) Multiple gene segments―which gene segments are put together.

2) Palindromic nucleotide addition ― templated nucleotide addition between joints.

3) Exonuclease trimming―sometimes occurs at junctions, losing nucleotides and changing reading frames.

4) Non-templated N nucleotide addition―mediated by TdT activity.

5) Combinatorial diversity – the same heavy chain can combine with different light chains etc

Question 60

What is allelic exclusion and receptor editing and why are they important?

Answer 60

Allelic exclusion ensures that each B cell synthesizes only one heavy and one light chain. Heavy chains are recombined and expressed first. Expression of a functional heavy chain shuts down
recombination machinery temporarily.

Receptor editing of potentially autoreactive receptors occurs in light chains to prevent antibody binding to self. Recombination machinery can be turned back on and is a last-ditch effort to salvage or inactivate the rearrangement.

Question 61

What known features of the TCR did Hedrick, Davis, and colleagues use in their quest to isolate the TCR gene? Why was the TCR harder to discover than the BCR and how was it done?

Answer 61

The BCR has a secreted version (antibody). This secreted protein could be used to generate
antibodies specific for the BCR that were useful in enriching it.

The TCR is a membrane bound protein with no secreted form, making the TCR discovery much more difficult to discover.

**Using the fact that it is membrane-bound, they isolated membrane-bound polysomes and used the RNA associated with the polysomes to generate cDNA probes specific for the membrane bound receptor genes. **

Question 62

Which TCR chain was discovered first and is it closer in structure to the antibody heavy or light chain?

Answer 62

The TCR β chain was discovered first and is similar to the BCR heavy chain with VDJ regions.

The TCR a chain genes are similar to the BCR light chain with only the VJ regions.