Immunoglobulins Flashcards
clones
individual B cells, with their unique B-cell receptors, are referred to as clones
clonal expansion
when a B cell is activated, ONLY the specific clone that matches the antigen is replicated
beneficial functions of immunoglobulins (antibodies)
- neutralize toxins or viruses (prevent it from binding to its receptor)
- aid in the uptake and clearance of bacteria (opsonization)
- facilitate complement activation
negative functions of immunoglobulins (antibodies)
- cause autoimmunity
- cause allergies and other hypersensitivity reactions
basic structure of an immunoglobulin
-composed of 2 IDENTICAL heavy chains (50 kDa) AND 2 identical light chains (25 kDa)
*joined by disulfide bonds (inter-chain and intra-chain)
-composed of a VARIABLE region and a CONSTANT region (Fc)
*sialylation (addition of sialic acid) of the constant region
variable region of an immunoglobulin
*antigen binding site (formed by both the heavy AND light chains)
Fc region of immunoglobulins
the four Cs:
Constant
Carboxy terminal
Complement binds here
Carbohydrate side chains
Fab region of immunoglobulins
*fragment antigen binding site; at N terminus of the immunoglobulin
*where unique immunoglobulin binds to an antigen
*each BCR has a unique Fab region
**antigen binding requires contact with BOTH the heavy and light chains
antibody
the secreted form of the B cell receptor (BCR), also referred to as an immunoglobulin
*recognize and bind specific antigens via Fab
antigen
the specific molecule(s) of a pathogen to which an immune response is directed
epitope
the specific part or feature of an antigen which is RECOGNIZED by an antibody
how do antigens and antibodies interact
all of the NON-covalent forces
[electrostatic, hydrogen bonds, van der waals, hydrophobic]
do antigens and antibodies use covalent bonds to interact
NO!!
just non-covalent bonds
linear epitope
a type of antigen:antibody interaction in which amino acid residues are ADJACENT in the polypeptide chain
discontinuous epitope
a type of antigen:antibody interaction created from amino acid residues located in DIFFERENT PARTS of the polypeptide chain
*antibodies responding to tertiary protein structure/folding
sources of generation of antibody diversity
- combinatorial diversity
a) multiple germ line gene segments (V, D, J)
b) multiple heavy and light chain pairings - junctional diversity
- somatic hypermutation
combinatorial diversity: V(D)J recombination
-light chain: V and J segments
-heavy chain: V, D, and J segments
*RAG-1 and Rag-2 (components of VDJ recombinase) mediate gene rearrangements
*once the rearrangement occurs, it is permanent, and the DNA in that B cell is different than the DNA in other cells!!
*occurs during B cell development in the bone marrow
which chain (light or heavy) gets rearranged first during V(D)J recombination
heavy chain gets rearranged first
RAG-1 and RAG-2
components of the VDJ recombinase that are critical for rearrangement of genes in B cell development
combinatorial diversity: multiple heavy and light chain pairings
virtually any of the heavy chains can be paired with any of the light chains
junctional diversity
RANDOM addition of non-template nucleotides at the junctions between gene segments (V/D/J) upon gene rearrangement by terminal deoxynucleotidyl transferase (TdT)
terminal deoxynucleotidyl transferase (TdT)
the enzyme responsible for junctional diversity during B cell development
*TdT deficiency greatly reduces diversity of immunoglobulins
where does somatic hypermutation occur
in the spleen or lymph nodes (NOT in the bone marrow) AFTER REARRANGEMENT has created a functional BCR
somatic hypermutation
*within the V component of the Fab region of both light and heavy chains, there are 3 hypervariable regions [3 in each of the chains, so 6 total that antigen is contacted by], which undergo point mutations with high frequency
*requires activation induced cytidine deaminase (AID)
*continues to happen in subsequent generations when exposed to antigen, all in an effort to get a more tightly bound antigen-antibody complex
hypervariable region 3 (HV3)
the most diversity is observed in this region because it is at the junction of V and J, so junctional diversity (by Tdt) can occur here in addition to somatic hypermutation
hypervariable regions and affinity
PROPORTIONAL: the affinity of an immunoglobulin for an antigen will increase as the number of hypervariable regions that participate in epitope binding increases
what part of the immunoglobulin determines its class (and subclass)
the heavy chain, specifically the Fc (constant) region; can be changed (class-switching)
isotypes
immunoglobulins of each class (IgG, IgM, etc)
which immunoglobulin classes are monomeric
IgG, IgD, and IgE
(IgM = pentameric)
(IgA = dimeric)
functions & characteristics of IgM
*produced in the immediate response to an antigen (FIRST antibody produced)
* pentameric when secreted to enable avid binding
*fixes complement
*does NOT cross the placenta
*found on surface of naive B cells
functions & characteristics of IgD
*found on surface of naive B cells and in serum
*unclear function
functions & characteristics of IgG
*main antibody in secondary response to an antigen
*MOST ABUNDANT ISOTYPE IN SERUM
*fixes complement
*opsonizes bacteria
*neutralizes bacterial toxins and viruses
*ONLY ISOTYPE THAT CROSSES THE PLACENTA
functions & characteristics of IgE
*binds mast cells and basophils
*cross-links when exposed to allergen, mediating IMMEDIATE (type 1) HYPERSENSITIVITY through release of inflammatory mediators (like histamine)
*contributes to immunity to PARASITES by activating eosinophils
*LOWEST CONCENTRATION IN SERUM
functions & characteristics of IgA
*prevents attachment of bacteria/viruses to MUCOUS MEMBRANES
*dimer when secreted
*released into SECRETIONS (tears, saliva, mucus) and breast milk
which immunoglobulin(s) enhance phagocytosis via opsonization
IgG
which immunoglobulin(s) neutralize toxins
IgM, IgG, IgA
which immunoglobulin(s) prevent adherence of pathogens
IgM, IgG, IgA
which immunoglobulin(s) activate CLASSIC complement system
IgM**, IgG
which immunoglobulin(s) transport across placenta
IgG
which immunoglobulin(s) cause sensitization of mast cells (allergic responses)
IgE
which immunoglobulin(s) are active at mucosal surfaces
IgA**, IgG
which immunoglobulin(s) are antigen receptors on naive B cells
IgM, IgD
isotype switching - overview
- initial phase: naive mature B cells is activated & predominant circulating antibody is IgM
- B cells switch to producing other isotypes, with IgG being the predominant circulating antibody
- during secondary responses, rapidly activated memory B cells secrete isotypes OTHER THAN IgM
the change from IgM to IgG or other isotypes does NOT
-involve additional heavy chain V/D/J or light chain V/J rearrangements
-change the heavy chain variable region or the light chain
-change the antigen specificity
class switching - detailed
- B cell as antigen-presenting cell:
a. receptor-mediated endocytosis
b. antigen presentation on MHC II and recognized by TCR on helper T (Th) cell - CD40 receptor on B cell binds CD40 ligand (CD40L) on Th cell
- Th cell secretes CYTOKINES that determine Ig class switching of B cell
where does class switching of B cells occur
in the GERMINAL CENTERS of lymph nodes or spleen (pale areas within lymph node follicles)
which happens first: class switching or somatic hypermutation?
class switching occurs prior to somatic hypermutation
is the production of IgD from IgM dependent on class-switching?
NO; alternative splicing of a single primary transcript encoding the mu or delta constant regions is responsible for the appearance of IgD
anti-isotypic antibodies
antibodies against the constant regions of heavy chains
*can be used to distinguish different classes of immunoglobulins
enzyme-linked immunosorbent assay (ELISA)
used to identify SPECIFIC PROTIENS in a solution
1. antigen (pathogen or protein) binds to plastic plate; excess washed off
2. test antibody is added and plate washed
3. ligand to test the antibody is added
4. substrate is added and a colored product is produced by enzyme reaction; amount of color is proportional to the amt of antibody bound to antigen
monoclonal vs. polyclonal antibodies
*most antibody responses are polyclonal (a range of B cells will make different antibodies against various epitopes of a given antigen
*a monoclonal antibody represents one unique immunoglobulin produced by a single B cell clone
monoclonal antibody production
- immunize a mouse with antigen of choice; remove the spleen when the mouse is making an antibody response
- fuse the immune spleen cells with a myeloma tumor cells
- culture the cells in selective medium so only fused cells survive
- grow cells in individual culture plate wells
- the clones (hybridomas) are immortal producers of the desired monoclonal antibody that can be used as a therapeutic agent
hyper-IgM syndrome
-due to mutated CD40 ligand on helper T cells (leading to lack of class-switching)
-X-linked recessive
-presents as severe pyogenic (pus-forming) infections in early life
-labs show normal or increased IgM with severely decreased IgG, IgA, and IgE and absence of germinal centers in lymph nodes
activation induced cytidine deaminase (AID)
the enzyme that is critical for somatic hypermutation during B cell maturation
