1 antibody structure Flashcards
what are antibodies and what cell type produces them?
- antibodies are the effector arm of humoral immunity
- they are produced by B lymphocytes and bind to antigen
B cells and T cells are both types of?
lymphocytes - part of the adaptive immune system
difference between adaptive and innate immune systems?
adaptive is faster and has memory
role of T cells?
- cytotoxic
- help other cells in the immune response
how can innate immunity can be divided?
humoral or cellular
humoral immunity?
mediated by macromolecules found in extracellular fluids such as secreted antibodies, complement proteins, and certain antimicrobial peptides.
cell mediated immunity?
doesn’t involve antibodies, but rather involves the activation of phagocytes, antigen-specific cytotoxic T-lymphocytes, and the release of various cytokines in response to an antigen.
what is the surface bound AB and what happens upon binding?
BCR
-upon binding, the ab signals internally to stimulate the B-cell to proliferate and produce secretory antibody
how do the different clones of B cells differ?
they each have an individual specificity for antigen
how many chains is an antibody made of?
4 chains
2 light and 2 heavy
The binding site of the antibody involves all 4 chains.
describe antibody structure
4 polypeptide chains: 2 identical heavy & light chains
held together by S-S bonds
glycosylated to allow for specific confirmation
Fab region - determines binding specificity
Fc region - determines function
Fab region
antigen binding region, composed of the variable regions of the heavy and light chain
determines the specificity and the affinity and avidity of the interaction with antigen.
proteases split the antibodies into?
2 parts - fab and fc regions
Fc region
Confirms the functional properties of antibody
Recognised by FcR (cell receptors) and binds complement (effector molecule of the immune system)
how are antibody classes defined?
defined by the heavy chain constant region
9 (sub) classes of antibody (in humans) determined by:
4x γ
1x μ
2x α
1x δ
1x ε
what antibody is a pentamer?
IgM, hence the large molecular weight
what antibody is a dimer?
sIgA
the majority of our serum contains what antibody?
Majority in the serum is IgG
-IgG has a higher half life, its is stimulated when you have been primed against an antigen
different antibody functions?
- immune complex, complement classical pathway (C1q)
- antigen binds to fab2 regions forms complex which complement can bind to - opsonisation + phagocytosis
- antibody binds to antigen, and antibody can bind to the Fc receptor on phagocytic cells - target cell
- Fc receptor on killer cells - infected cell with antigen on surface - primed cell, sensitisation
- IgE produced in first encounter, second encounter the IgE binds to antigen and mast cells degranulate, histamines and anaphylaxis
IgM
5 identical IgMs linked together by a J chain
They each have a low affininty, but when you put them together you get a high avidity
Heavy chain encoded by μ gene
Constitutes ~ 10% serum Ig
Major antibody in primary response - first one made
4 CH domains, stabilised by J-chain
Monomer forms the BCR on most B cells in association with Igα and Igβ chains
IgG
Because IgG is small and monomeric, it can cross the placenta and protect fetal development
Monomeric, heavy chain γ gene,3 CH domains. Major circulating Ig (70-75%) Major antibody in secondary response 4 sub-classes G1; G2; G3; G4 Activates complement (Classical) Opsonin (FcR)
IgA
Range of polymers (shapes) from monomer to pentamer - mostly monomer and dimer in secretions.
1 J chain per polymer - the J chain stabilises it
Heavy chain encoded by α-gene
15-20% circulating Ig
Epithelium adds secretory component which protects against degradation due to proteolysis; aids release into mucous.
2 subclasses IgA1 in serum IgA2 in mucous
Found in tears, milk, saliva, sweat etc
Protection of external surfaces- first line of defence
Can get localised mucosal response, different to systemic response - It is secreted onto the mucosa to protect against enteric pathogens
Does not activate complement (by classical pathway)
IgA secretion
IgA dimers bind to Poly-Ig receptors on basolateral surface of the epithelial cells.
Complex endocytosed and still bound to membrane.
Vesicles fuse with the luminal surface and receptor cleaved by proteolysis.
Dimer released attached to secretory component
IgD
Heavy chain coded by δ gene
Monomer, very like IgG but slight differences in heavy chain constant region
<1% serum Ig
Has specific antigen binding activity but NO effector functions
Sensitive to proteolytic degradation and heat
Expressed on the surface of B cells with monomeric IgM
Involved in antigen triggered B-cell differentiation
IgE
Heavy chain coded by ε-gene
Heavy chain has 4 constant domains (looks like monomer IgM)
Trace levels in serum (elevated in allergic or heavy parasitic infection)
Majority bound to mast cells and basophils through high affinity FcεR1
Key to allergic response
Important role in parasitic infections, mast cells very effective at killing multicellular organisms
what do most B cells express?
monomeric IgM and IgD
- Both Ig have the same specificity on each individual cell
- 10% of circulating B cells express IgG, IgA or IgE (after memory and differentiation)
- Some tissue bias, eg mucosal B cells express IgA
structure of surface bound Ig?
very short intracytoplasmic tail, thus is associated with ‘accessory’ molecules to form the BCR
Igα (CD79a) and Igβ (CD79b) - (similar to TCR and CD3)
BCR binding effects
Ligation of the BCR results in ITAM phosphorylation (Immunoreceptor Tyrosine-based Activation Motifs).
Leads to downstream cascade of events resulting in plasma cell differentiation and antibody production.
CD79a and b are associated with the B cell receptor to cause the transmission and activation of B cells
Outcome of BCR
ligation
When a B cell becomes activated, it can become a plasma cell or a memory cell
Clonal expansion: B cell becomes activated and becomes a clone of cells with exactly the same BCR specificity
A small proportion of cells remain as long-lived memory cells
The majority of cells become effector cells (plasma cells) which produce antibody, lots of it!
Plasma cells have a limited life-span and apoptose after a few days
Class switching
First Ab response is IgM
Secondary response is switched to other classes of Ab, especially IgG
Mutate their variable region gene sequences- somatic hypermutation
Affinity maturation
Needs T cell help
role of mutation in class switching?
Ab’s mutate their binding site in a random fashion to increase the affinity of the binding site for the antigen
In the secondary response – we get a different type of antibody and that has a greater capacity for binding its antigen
Monoclonal antibodies
The single most important immunological discovery of the century!
Inject the antigen you want to make antibodies against into the animal, and harvest the B cells from spleen after a few weeks.
Fuse B cells with myeloma cells (constantly
growing lymphoid cells). Myeloma cells are immortal cells from a B cell tumour, they lack the HGPRT gene.
Ethylene glycol is used to help fuse - melts membranes. Forms hybidroma which are the only cell type that can grow in the HAT medium.
The hybridoma will spontaneously grow like myeloma. And also produce Ab.
examples of new checkpoint inhibitors:
Ipilimumab:- anti-CTLA-4
Nivolumab:- anti-PD1
problems with monoclonal antibodies?
Constant region heterogeneity
Most early McAb were murine
Anti-antibodies
Inflammation
solutions: Chimeric antibodies: Murine antigen recognition sites grafted to human constant regions…. Humanised.
Fully humanised antibodies: ‘phage display (in vitro McAb) and humanised mice!
main functions of monoclonal antibodies
anti inflammatory or anti cancer
Infiximab used for athrities (anti TNF-alpha).
TNF-a = inflammatio, partcuilalry in joints. Helped
athritis a lot. Mops up inflam mediators, causing disease, (so doesn’t realy stop disease)
Rituximab binds to molecule found on B cell, to treat B cell tumour.
Herceptin – binds to receptor for growath factor.
Zumab – humanized molecule antibody has a bit of change, to look like human molecule.
Ximab at the end = chimeric antibody. (part of human and mouse antibody – they do this to stop it
from becoming an antigen itself)
new checkpoint inhibitor: Nivolumab is anti-PD1, stops switching immune system off.
affinity vs avidity
affinity is the strength of a single interaction between an antibody and its epitope
Avidity is the sum of the different affinities (some antibodies will be multimeric and will have several binding sites)
