Immunology - E3 Flashcards
Features of Innate Immunity
Prevent infection or eliminate a pathogen.
- Present in all individuals at all times
- Earliest response to infection (minutes/hours)
- Recognizes groups of similar pathogens (not “antigen-specific”)
- Not increased with repeated exposure to a pathogen (no memory)
Examples of mechanical barriers (innate immunity)
Skin / mucosa
Movement of mucus by cilia
Examples of biologically active substances (innate immunity)
- Anti-microbial proteins (skin, mucosa)
- Cytokines (IL-1, IL-6, TNF and others) *fever, depression, and anorexia
- Acute phase proteins: C-reactive protein (CRP) etc.
- Activation of Complement (Alternative and Lectin Pathways)
What are cytokines?
“hormones of the immune system” (regulate immune response).
Cause fever, increased WBC, increased CRP, and recruit inflammatory cells to the site of infection. (Exp. IL-1, IL-6, TNF)
What is C-reactive protein (CRP)
Example of an acute phase protein. Levels rise via the liver in response to inflammation.
Innate immunity (Cellular) results in the activation of…
Activation of leukocytes (white blood cells):
Dendritic cells (DC) –> have PRR
Macrophages (Mf) –> phagocytosis, have PRR
Neutrophils –> phagocytosis, have PRR
Natural killer cells (NK cells) –> cytotoxicity
Mast cells and Basophils –> inflammation
Eosinophils
Macrophages
Large, mononuclear phagocytic cells that are present in most tissues.
Macrophages are derived from blood monocytes (2-6% of WBCs).
Neutrophils
(50-60% of WBCs), are phagocytic cells, also known as:polymorphonuclear neutrophils (PMN)
A major function of neutrophils is to enter infected tissues to engulf and kill extracellular pathogens, especially bacteria
Eosinophils
(1-4% of WBCs)
- kill parasites that too large to be ingested by phagocytes.
- release substances that are toxic to helminths.
- involved in allergic responses.
Mast cells
- Found in connective tissues throughout the body.
- Involved in response to parasites (especially helminths) and allergic responses.
Basophils
(0.5-1% of WBCs) found in the blood and are thought to have a similar function as mast cells.
Dendritic cells (DC)
Found in tissues and function todetect infection and elicit an early innate response
What are pattern recognition receptors (PRR)?
Cells of the innate immune have receptors for pathogens called pattern recognition receptors (PRR).
Not specific for a particular bacteria species, recognize a GROUP of pathogens.
The innate immune system detects infection using about a few dozen.
What is the microbial product recognized by PRRs?
PRRs recognize a pathogen associated molecular pattern (PAMP)
- Exp. dendritic cells and macrophages are activated when TLR-4 (a PRR) recognizes LPS (a PAMP) from gram negative bacteria.
ADAPTIVE (ACQUIRED) IMMUNITY : Host defenses mediated by the_______________ and differentiation of antigen-specific lymphocytes (______________).
Host defenses mediated by the clonal expansion and differentiation of antigen-specific lymphocytes (B cells and T cells).
Features of ADAPTIVE (ACQUIRED) IMMUNITY
Requires sensitization by antigen (Ag)
Develops over days/weeks
Response is antigen-specific
Results in immunological memory
Adaptive immune responses can be classified as:
Humoral Immunity or Cell-mediated Immunity (CMI)
Humoral Immunity
- mediated by antigen-specific antibodies (Ab) produced by activated B cells (plasma cells)
- Antibodies can be transferred to non-immune (naïve) recipients by immune serum (antiserum).
Cell-mediated Immunity (CMI) are adaptive immune responses primarily involving _________.
Antigen-specific T lymphocytes (T cells).
Can CMI can be transferred to naïve recipients by T cells?
Yes, but not by immune serum.
Clonal expansion
Following activation by antigen, a B cell proliferates and produces a large clonal population of long-lived memory B cells and antibody-secreting plasma cells.
Shape that Ab recognizes and binds is an _______
Epitope
Explain how Ab response is polyclonal
On any antigen –> multiple shapes that are foreign and different Abs will recognize those epitopes.
Exp. Virus has many proteins, those proteins have many epitopes, and there are many B-/T- cells responding to those different shapes on virus.
^(Polyclonal, many clones of B-cells and T-cells will respond to any infectious agent).
What are the primary lymphoid organs?
Where B and T cells undergo differentiation culminating in the expression of antigen-specific receptors.
B cells – bone marrow
T cells- thymus
What are secondary lymphoid organs?
- where lymphocytes encounter and respond to antigens.
- designed to trap antigen and facilitate antigen contact with lymphocytes.
Exp. Adenoid, Tonsil, Lymph nodes, Spleen, Peyer’s patches
Describe the structure of an antibody
Two H-chains (heavy chains) and two L-chains (light chains).
For a particular antibody, the H-chains are identical and the L-chains are identical. (2 identical antigen-binding sites)
What is the Fab region of an Ab?
Fab = ““fragment antigen binding”
- 2 Fabs/antibody molecule monomer, so each monomer contains two binding sites for an antigen.
- 2 Fab regions are identical
What is the Fc region?
Fc = determines the effector function(s) that will be activated by the antigen-antibody complex.
What about the structure of an Ab makes it specific?
N-terminal of heavy & light chains differs Ab to Ab (variable region, what makes Ab specific), rest is constant.
What is the antigen-binding site?
Variable region from one light chain and the variable region from one heavy chain combine together to form the shape that recognizes and binds antigen (antigen binding site).
Multiple Myeloma
Plasma cell (effector B cell) becomes neoplastic, growing out of control and crowding out the bone marrow.
Plasma cells –> produce large amounts of specific Ab. (normal Ab in structure, but is produced in abnormally large amounts.)
The AB produced by the neoplastic cells (a myeloma protein) is monoclonal (produced by a single clone)
Describe the Complementarity Determining Region (CDR) of an Ab
W/i variable regions –> 3 regions are hypervariable.
(Heavy chain has 3 CDRs, and light chain has 3 CDRs)
- These regions are up top, touching the antigen.
In mammals, there are five classes of Ig based on the ______________________________.
Sequence of the heavy chain constant region.
Constant region of heavy chain decides what the FC is (the effector part of the molecule).
A particular antibody will have one of the ___ possible _____-chain constant regions.
IgG1 (gamma-1)
IgG2 (gamma-2)
IgG3 (gamma-3)
IgG4 (gamma-4)
IgA1 (alpha-1)
IgA2 (alpha-2)
IgM (mu)
IgD (𝛿)
IgE (epsilon)
- Everyone makes all 9 of these isotopes.
The light chains associated with an antibody will be either ______ (k) light chains or _______ (l) light chains.
kappa (k) light chains or lambda (l) light chains
Two Ig classes can form polymeric Igpentameric IgM and dimeric IgA. Polymeric Abs have a single _______ protein bound to them.
J-chain protein
___ producing plasma cells always make pentameric ______ producing plasma cells makeeither monomeric ___ or dimeric ___
IgM producing plasma cells always make pentameric IgM
IgA producing plasma cells make either monomeric IgA or dimeric IgA
B-Cell Antigen Receptor (BCR)
All B cells have an antigen-specific receptor consisting of a monomeric Ig molecule (two H-chains and two L- chains)
Membrane-bound Ig is associated with a signaling molecule consisting of two proteins…
a heterodimer of _______ and ______.
Ig-alpha and Ig-beta.
Naïve B cells have BCRs of both___ and ___ classes.
The BCR of a memory B cell will be___, or ___ or ___ (never more than one class).
Naïve B cells have BCRs of both IgM and IgD classes.
The BCR of a memory B cell will be IgG, or IgA or IgE (never more than one class).
There are about 10,000 of these on each B-cell (all are identical on any given B-cell).
Describe agglutination (clumping antigen)
Because IgM is a pentamer (10 antigen-binding sites) this class is very effective at clumping antigen.
These large aggregates are more readily removed by phagocytes.
Describe neutralization of virus (or toxin) by antibody
Usually…
- Virus binds to receptors on cell surface.
- RCME of virus
- Acidification of endosome after endocytosis triggers fusion of virus with cell and entry of viral DNA
Ab binds to the receptor on the virus –> blocks the binding to virus receptor, thus block fusion event (viral neutralization. IgG and IgA are the most effective neutralizing Abs.
Describe inhibition of Bacterial Adhesion by antibody
Usually…
- Colonization of cell surface by bacteria via bacterial adhesions
- Some bacteria become internalized and propagate in internal vesicles
Ab against adhesions block the colonization and uptake of bacteria
What is opsonization?
Coating of a microbe or other particle with a substance that makes the microbe more readily phagocytosed.
If put IgG on a microbe, it can be recognized more efficiently. (opsinization).
IgG is an important opsonin. (Free Ig does not cross-link Fc receptors, but aggregation of Ig on bacterial surface allows cross-linking of Fc receptors and thus activation of macrophage, phagocytosis, destruction)
Describe the mechanism of opsonization
Phagocytic cells (primarily macrophages and neutrophils) have Fc receptors that bind the Fc region of Abs, thus ingesting Ab-coated pathogens efficiently.
(Fc=part of the Ab that directs its “biological activity”. Here, Fc region facilitates phagocytosis of antigen by phagocytic cells.)
- When IgG binds to bacteria, the Fab is the portion of the Ab that actually binds the bacteria cell surface.
- The Fc portion of the Ab “sticks out” from the surface of the bound bacteria. Neutrophils and macrophages have Fcg-Receptors (FcgR) that will bind to the IgG-coated bacteria.
Describe Antibody-dependent cellular cytotoxicity (ADCC)
How Abs can enhance an innate effector function.
Antibodies can be made to viral antigens on the surface of infected cells or to tumor antigens on the surface of neoplastic cells.
- Ab binds antigen on the surface of target cells.
- The presence of FcgR (receptor for the Fc of IgG) on NK cells facilitates the ability of NK cells to recognize aberrant cells that are coated with IgG.
- Cross-linking of Fc receptors signals the NK cell to kill aberrant cell.
- Target cell dies via apoptosis.
How are NK cells an important component of innate immunity?
In contrast to B cells and T cells, NK cells do NOT express antigen-specific receptors.
NK cells recognize aberrant cells via receptors that recognize general features of infected or transformed cells. (the NKs express an Fc receptor and so induce some virus-infected or tumor cells to undergo apoptosis.)
Describe the immune response of mast cells and what this is an important protection against.
The inflammatory response resulting from mast cell degranulation functions in immunity to worms.It is also a key element in allergic reactions (mast cells release histamine).
Mechanism:
- Fc-epsilon recognizes IgE antibody.
- Multiple antigen cross-links bound IgE, causing release of granules which help get rid of bug.
What is unique about mast cell activation as opposed to opsonization and ADCC?
In contrast to opsonization and ADCC, mast cell FceRs bind IgE before IgE binds with antigen.
IgE already bound to FceRs on mast cells can then bind to antigen.
Mast cell pre-arms itself with IgE.
Antibody functions - who does it BEST?
Ag-specific receptor (BCR):
Agglutination:
Neutralization:
Ag-specific receptor (BCR): all isotypes
Agglutination: IgM
Neutralization: IgG and IgA
Antibody functions - who does it BEST?Complement activation:
Complement activation: IgM and IgG
(The complement system is a series of proteins that mediates host defense against various extracellular pathogens, especially bacteria)
Antibody functions - who does it BEST?Opsonization:
ADCC:
Mast cell activation:
Opsonization: IgG
ADCC: IgG
Mast cell activation: IgE
Which are the FcR-dependent functions of Abs?
Opsonization
Antibody-dependent cellular cytotoxicity (ADCC)
Mast cell activation
Which immunoglobulin is most highly concentrated in tissue fluid in blood?
IgG is the major immunoglobulin in the blood and tissues. Because IgG is present as monomers, it can diffuse out of capillaries into adjacent tissues.
Describe the distribution of IgM in the body.
IgM pentamers, because of their large size, are mainly confined to thebloodstream.
Describe the distribution of IgA in the body.
IgA dimers: major secretory immunoglobulin, found in mucosal secretions (milk, saliva, tears).
Large surface area of mucous membranes (digestive tract, respiratory tract, urinary tract, etc.) –> more IgA antibody produced each day than any other class.
Describe the importance of IgG in newborns
Babies are born with maternal IgG (the only antibody class that can cross the placenta.) IgG has a “half-life” of about 3-4 weeks, the longest of all antibody classes.
Thus, maternal IgG provides passive immunity to the newborn that is protective for 2-3 months.
Standard practice for tdap vaccine to be given (want baby to have high concentration to prevent neonatal tetanus)
The predominant Ig in secretions (saliva, milk) and at mucosal sites?
Secretory IgA (SIgA).
Dimeric IgA but MORE –> in addn to 2 IgAs, it has secretory component protein.
How does secretory IgA pass across epithelial cells to reach mucosal surfaces or enter secretions?
- Dimeric IgA binds the polymeric Ig receptor (pIgR) at the basolateral membrane.
- It is internalized and transported across the cell through the cytoplasm (transcytosis).
- Dimeric IgA is then released from the apical surface of the cell onto the mucosa.
- pIgR and dimeric IgA bind covalently and permanently. Only way to release IgA is a chainsaw (enzymatic cleavage).
- Most dimeric ends up stuck to the receptor –> secretory component + dimeric IgA = secretory IgA.
What is a “common mucosal immune
system”?
A “common mucosal immune system” is the concept that immune responses that occur at one mucosal site result in IgA secreting cells migrating, via the lymphatics and blood,
to distant mucosal sites.
What is an scFv, and why is it useful?
A scFv is an antigen-binding fragment (Fv) of an Ab engineered by coupling a VL domain and VH domain via a flexible polypeptide “linker.”
Thru genetic engineering, you can create a single chain Fv (just Ag binding site (VH and VL) and produce it as single polypeptide chain – so it can fold efficiently.
You can stick this on a toxin –> an immunotoxin results. Can use this to target tumor cells.
What is V(D)J recombination?
The DNA rearrangement mechanism that creates many billions of BCRs and TCRs using relatively few Ig and TCR genes
Which two proteins mediate V(D)J recombination and where are they found?
RAG-1 and RAG-2 mediate rearrangement of Ig genes in developing B cells (bone marrow) TCR genes in developing T cells (thymus).
Describe light-chain gene rearrangement.
One V segment gene and one J segment gene come together in the genome of a differentiating B lymphocyte to create a complete V-J gene unit that, together with the constant (C) region gene, codes for an entire antibody light chain.
An individual B cell will rearrange and express only one light chain using either the kappa locus or the lambda locus (never both).
What are the two main differences between light and heavy chain gene rearrangement?
- The variable region of the heavy chain is coded for by three gene segments: a variable (V) segment, a diversity (D) segment (20 of these in humans), and a joining (J) segment.
- The presence of multiple genes coding for the constant region of the heavy chain in the heavy chain Ig locus; one C gene for each of the nine human Ig isotypes.
How do naïve B cells produce IgM and IgD simultaneously?
C-mu and C-𝛿 constant region genes are the closest to the developing B cell’s unique VDJ.
C-mu and C-𝛿 correspond to IgM and IgD heavy chain constant regions.
Via RNA splicing, the B cell can express BOTH IgM and IgD and both antigen receptors will have exactly the same antigen specificity.
What are the three means of generation of Antibody Diversity?
- Germline: multiple inherited V, D and J
- Combinatorial: V + D + J and H + L
- Junctional: imprecise joining of V, D, and J gene segments by exonucleases and TdT.
(TdT = terminal deoxyribonucleotidyl transferase)
Describe the mechanism of TdT (terminal deoxyribonucleotidyl transferase) and how it contributes to junctional Ab diversity.
When gene segments are excised for V(D)J recombination, exonucleases randomly “chew back” a variable number of NTs from the ends of the joining segments.
TdT randomly adds entirely new NTs to the ends of the segments prior to their joining –> makes the junctions between V, D, and J gene segments highly variable –> important for the creation of diversity at the antigen binding site.
Describe Somatic Hypermutation (SHM) and how it contributes to Ab diversity.
*Occurs only in B-cells
POST ANTIGEN ACTIVATION, increase in the affinity of antibody for the antigen is observed –> Somatic hypermutation (affinity can increase 100 to 1000-fold over time)
Refers to random point mutations that occur in the variable region (antigen-binding sites) of immunoglobulin heavy and light chains in the B cells of an expanding clone.
(“survival of the fittest” = B cells making high-affinity BCR)
B cells and T cells use V(D)J recombination to generate antigen-specific receptors
B cells rearrange _____
T cells rearrange _____
B cells rearrange Ig loci
(Igh and either Igk or Igl)
T cells rearrange TCR loci
(TCRa and TCRb, or TCRd and TCRg)
A complete deficiency of either RAG1 or RAG2 causes what?
Absence of both B cells and T cells and a complete loss of adaptive immunity. Can’t make receptors.
Class Switch Recombination (CSR)
After activation by antigen, a B cell can “switch” to produce different classes of antibody molecules, such as IgG, IgE, and IgA, while still retaining the same antigen specificity.
Same heavy chain VDJ rearranged genes are moved next to a different constant region gene (either Cg, Ca, or Ce). ONLY the heavy-chain C region is changed, thereby preserving the antigen specificity of the B cell.
While individual mature (naïve) B cells can produce _______________ on their cell surfaces, after class switching, an individual B cell can _________________.
While individual mature (naïve) B cells can produce both IgM and IgD on their cell surfaces, after class switching, an individual B cell can only produce one of the “switched isotypes” (i.e. IgG or IgE or IgA).