3 - Antigens and Antibodies Flashcards
1
Q
features of an adaptive immune response
A
- specificity
- immunologic memory
- diversity
- self-regulation
- discrimination of ‘self’ from ‘nonself’
2
Q
Specificity
A
- Adaptive immune responses are exquisitely specific.
- Immune responses are generated toward determinants/epitopes.
- Specificity is due to lymphocytes having cell membrane receptors .
- Each lymphocyte has just one specificity!!
3
Q
Immunologic memory
A
- feature of an adaptive immune response
- Faster and more vigorous immune response upon re-exposure to antigen:
- – Due to the presence of memory cells
- – Memory cells → more sensitive to stimulation by antigen than antigen-naive lymphocytes
- – increases each time exposed
- takes about 7-10 days to develop when exposed to new antigen (why we need innate immunity!). Faster for second exposure
4
Q
Diversity
A
- feature of an adaptive immune response
- Pre-existing antigen-specific lymphocytes capable of reacting to ~1x109 antigens
- Due to the expression of cell surface receptors that can react with a diverse number of antigens
5
Q
self-regulation
A
- feature of an adaptive immune response
- All immune responses abate with time due to:
- – Removal of antigen, therefore no further immunologic stimulation
- – Activated lymphocytes dying within a short period of time by apoptosis
- – Regulatory immune mechanisms
6
Q
Discrimination of self vs. non-self
A
- feature of an adaptive immune response
- Adaptive immune responses are directed normally against foreign antigens, not self antigens
- Immune cells specific for “self” are usually destroyed or regulated
- Autoimmune disease states may occur when discrimination is lost
7
Q
Immunogenicity
A
- Properties that promote an immune response
8
Q
Antigenicity
A
Properties that allow a substance to react with an antibody; used loosely to describe an immunogen
9
Q
Haptens
A
- Small molecules that cannot induce antibody formation but can react with antibody that is specific for it:
- – Haptens must be coupled to a carrier molecule to induce antibodies
- – Therefore, a hapten is an antigen, but not an immunogen (this is the main distinction between these two, otherwise interchangeable, terms)
10
Q
Important features of immunogens
A
(things promoting good immune response)
- Size
- Internal complexity
- Degradability
- Foreignness
- Accessibility
11
Q
Important features of immunogens: size
A
> 10,000 M.W. = best immunogens
- can be too small to develop a good immune response
12
Q
Important features of immunogens: internal complexity
A
More complex → more immunogenic
13
Q
Important features of immunogens: degradability
A
Immunogen processing must occur
- processed into smaller chunks in order to promote immunity
14
Q
Important features of immunogens: foreignness
A
Tolerance to self antigens → must be foreign
- more foreign = better immune response (can tell easier that the antigen is not like you)
15
Q
Important features of immunogens: accessibility
A
Easy-to-reach areas (i.e. not buried) are more likely to induce an immune response (called immunodominant areas).
16
Q
Antigen conformation
A
- Determinant/epitope = Piece of an antigen the immune system can respond to (can be conformational or linear)
- Conformational determinants/epitopes - amino acid residues from different parts of the protein brought together in 3-D space
- – determinant lost by denaturation
- Linear determinants/epitopes - formed by adjacent/continuous amino acids
- – can be in regular globular protein and/or denatured
- Neoantigens - “new antigens“ formed by proteolysis, phosphorylation, etc.
- – see in tumor immunology
- Most antibodies that are generated against intact proteins will recognize conformational determinants, but some of the antibodies might recognize a linear peptide fragment of the original protein. However, T lymphocytes can only recognize linear determinants.
17
Q
types of antigens
A
- protein
- lipoprotein
- polysaccharides
- glycoproteins
- polypeptides
- nucleic acids
18
Q
*protein antigens
A
- source: serum proteins, microbes
- Most common antigen to which the body is exposed
- Most immunogenic
19
Q
source of lipoprotein antigens
A
Cell membranes
20
Q
source of polysaccharide antigens
A
bacterial capsules
21
Q
source of glycoprotein antigens
A
blood group substances
22
Q
source of polypeptide antigens
A
hormones
23
Q
source of nucleic acid antigens
A
cells, microbes
24
Q
Antibodies/immunoglobulins
A
- Terms used interchangeably
- Present in body “humors” (fluids)
- Glycoproteins that mediate their biological effects by binding to antigen in a very specific manner
- Specificity similar to that of enzymes and their substrates (like lock and key)
- Not present until stimulated by antigen
25
Locations of antibodies
- Surface of B lymphocytes (antigen receptors)
- -- Each B lymphocyte expresses one specificity
- -- Antigen-naive B lymphocytes have both IgM and IgD
- Blood plasma and tissue fluids contain large amounts of antibody
- Surface of mast cells/basophils contain receptors for IgE antibody
- Secretory fluids (mucus and milk)
26
antiserum
- Antibody-containing serum = "antiserum". Serum is the fluid portion of the blood after the cellular elements have clotted.
- -- Polyclonal antiserum - population of antibodies which (collectively) can bind to more than one particular antigen
- -- Monoclonal antiserum - antibodies which bind one specific antigen
27
Antibody titer
- “Antibody titer” = reciprocal of the last dilution of antiserum that still yields a demonstrable antibody binding reaction
- Example: An assay which measures antibody binding to an antigen gives a demonstrable reaction at an antibody dilution less than or equal to 1:32, but not a dilution of 1:64. What is the titer of the antibody?
- -- use the 1:32 dilution so titer is 32/1 = 32
28
antibodies in electrophoresis of serum
Electrophoretically separated serum proteins will migrate within bands.
- Most antibodies are found in the gamma fraction → "gamma globulins"
- Antibodies are found in other bands so “immunoglobulin” more accurate
- “Immunoglobulin” is synonymous with “antibody”
29
Basic antibody structure
- All antibodies → 2 identical heavy chains and 2 identical light chains
- Heavy chains connected together by covalent disulfide bonds
- Light chains connected to heavy chains by disulfide bonds → form the antigen binding site
- Heavy and light chains composed of constant (C) and variable (V) regions
- -- Alters amino acid sequence in V region for different antigen specificities
- -- Heavy and light chain variable regions form the antigen-binding site
- Variable regions subdivided into hypervariable and framework regions:
- -- Hypervariable regions on each chain form the binding surface. Varying a.a.’s in hypervariable region → different antigen specificities.
- -- Framework regions support the hypervariable regions.
- -- (Ex/ fingertips = hypervariableregions, fingers = framework regions, one arm = light chain, other arm = heavy chain, apple = antigen))
30
properties of antibody molecules
- IgG, IgA and IgD have a hinge region → flexibility of the antibody arms
- Isotypes expressed in two forms, secretory and membrane form (except IgD, which only has a membrane-bound form)
- Secretory forms of IgM and IgA have a J chain to allow formation of pentamers of IgM and dimers or trimers of IgA
- Isotype switching allows a single antigen specificity to be used for different biological functions
31
Proteolysis of antibodies
- Pepsin digestion results in a single F(ab')2 fragment, with two antigen binding sites (connected), and no surviving Fc fragment.
- -- can still cross-link antigens
- Papain digestion yields two Fab fragments (each having one antigen binding site) and one Fc fragment.
32
Antibody classification
- Antibodies can be divided into classes or isotypes based on differences in the amino acid sequence of their heavy chains.
- Isotypes or classes include immunoglobulin G (IgG), IgM, IgA, IgD, and IgE.
- -- Each class of antibody mediates a different biological effect.
- -- Each heavy chain is denoted by the Greek letter of its class (i.e. gamma, mu, alpha, delta, and epsilon).
- Certain classes can be further divided into sub-classes or sub-isotypes.
- -- IgG sub-classes include IgG1, IgG2, IgG3, and IgG4.
- -- IgA sub-classes comprise IgA1 and IgA2.
- monomers: IgG, IgE, IgD (aka GED)
- polymers (when secreted): IgA and IgM
- -- have J chains
- -- if present in B-cell membrane, are there as a monomer
- -- IgA = two connected by a J chain
- -- IgM = 5 connected by a J chain
33
IgG characteristics
- Most abundant immunoglobulin in normal serum (longer half-life)
- Secretory form = monomer
- Activates complement by the classical pathway
- IgG1 and IgG3 can opsonize to enhance phagocytosis (next slide)
- Can coat tumor cells or virus-infected cells to facilitate antibody-dependent cell-mediated cytotoxicity (ADCC) (next slide)
- Crosses placenta to convey protective immunity to the fetus (only one!)
- In mother's milk → taken up by the infant’s gut lumen and transported into the blood (so is IgA)
34
IgM characteristics
- This isotype accounts for about 10% of total serum antibody.
- -- Secretory form = pentamer
- -- Excellent activator of complement (even better than IgG)
- -- Predominate antibody produced in a 1o immune response (IgG is the main antibody produced in a 2o immune response)
35
IgA characteristics
- 15% of the total serum immunoglobulins
- Mediator of mucosal immunity (lungs, gut, UG tract)
- Present in tears, saliva, colostrum, and milk
- Monomeric in serum, but dimer in secretions
- IgA dimer held together by a J-chain (similar to IgM)
- Eosinophil-mediated ADCC of certain parasitic infections such as helminths (i.e. worms)
- Transported through mucosal epithelial cells (b/c made in lamina propria but need to get to the lumen)
- Coupled to “secretory piece” to protect IgA from proteolytic enzymes (in gut) and act as a "glue" to bind it to the mucus
- Secretory piece/component derived from epithelial cell Fc receptor that binds IgA (NOT from plasma cell)
36
IgE characteristics
<2% of the total serum immunoglobulins
Secreted as a monomer
Eosinophil-mediated ADCC of certain parasites (e.g. worms)
Binds to cell surface receptors for IgE on basophils and mast cells to mediate allergies and anaphylaxis. The IgE receptor is high affinity.
37
IgD characteristics
- Very low concentrations in serum (not secreted)
- Primarily on the surface of antigen-naive B lymphocytes (with IgM)
- Important in the transduction of signals across the plasma membrane to result in antigen-driven B cell activation
38
Affinity
- Strength of binding for antigen of one antigen combining site, i.e. one arm of an antibody.
- IgG has greater affinity than IgM. (b/c only looking at one arm)
39
Affinity maturation
Average affinity for a population of antibodies will increase with repeated immunization with an antigen
40
Avidity
- Overall strength of attachment which takes into account how many antigen combining sites the antibody has bound.
- IgM has greater avidity than IgG. (5 antibodies = 10 arms. lots of binding space)
41
Allotype
- Differences in the constant regions of antibodies (of the same isotype) between individuals due to the presence of multiple alleles of the constant region genes in the human population
42
Idiotype
- Collection of hypervariable regions contributed by heavy and light chains that form the antigen-binding site
- our immune system can produce as many idiotypes as antigens (about 1-10 billion)
43
antigen
- antibody-generating thing
| - each antibody recognizes only one antigen
44
Adjuvant
- Immunogenicity is ↑; something you can add that prolongs retention so that a more vigorous immune response can occur
- Common adjuvants: alum (used in vaccines), mineral oil, and lipids
