Introduction to antibodies and B lymphocytes Flashcards
Describe the antibody structure
Consists of 4 polypeptide chains
2-identical heavy chains (~50kDa) - Alpha and beta
2-identical light chains (~25kDa) - kappa and lamda
Chains held together by S-S bonds and multiple non-covalent interactions
Glycosylated
The N terminal ~ 110 amino acid residues of the light and heavy chains = variable regions
The C terminal ~ light and heavy chains forming the constant region
Describe the folding and domains in light and heavy chains
Each segment of -110 amino acids folds to form a compact domain (3D structure/ the immunoglobulin fold), which is stabilized through a multiple noncovalent interaction and a covalent intrachain disulfide bond. Thus:
- the light chain has two domains and an intrachain disulfide bond in each of the VL and CL domains;
- the heavy chain has four domains and an intrachain
disulfide in each of the VH, CH1, CH2, and CH3
domains.
Each disulfide bond encloses a peptide loop of 60–70 amino
acid residues.
What is the purpose of the immunoglobulin fold
provides for both stability and a multiplicity of structural and functional variant which may or may not have an overt immune function e.g.
TCR, MHC molecules, cellular receptors for antibodies and adhesion molecules
These molecules belong to the immunoglobulin supergene family (IgSF)
Antibodies are prototypes of the immunoglobulin superfamily
What defines the immunoglobulin superfamily members?
There is significant amino acid sequence homology between antibody domains which is reflected in a common conformational motif, referred to as the immunoglobulin fold. This characteristic fold defines the immunoglobulin
superfamily members.
What are antibodies produced and secreted by?
B lymphocytes/Plasma B cells
Describe the F(ab)2 region
F(ab’)2 contains the antigen binding region
Antigen recognition site composed of the variable region of both the heavy and light chain
Determines both the specificity and the affinity and avidity of the interaction with antigen
Hypervariable
Describe the Fc (crystallisable)region
Fc region confirms the functional properties of antibody - only occurs in few variants - heavy chain isotypes
Allows antibody to activate immune system
The Fc region:
- Interact and is recognised by the cell surface receptor - Fc Receptor (FcR)
- binds to the first component (C1q) of the complement system to initiate the classical pathway complement cascade
How do antibodies act as adapter molecules for immune effector systems
(1) Immune complexes can activate the complement classical pathway
(2) Antibodies bound to the surface of pathogens opsonise them for phagocytosis
(3) Antibodies bound to the cells can promote their recognition and killing by NK cells
(4) Antibody bound to Fc receptors sensitises cells so that they can recognise antigen, and the cell becomes activated if antigen binds to the surface antibody
how many classes and sub classes of antibodies are there?
How are they similar/ differ?
5 classes &
9 subclasses
of antibody
(in humans)
They differ in:
- size
- charge
- amino acid sequence
- carbohydrate sequence
Similar:
All antibodies are bifunctional except IgD. They:
- recognise and bind antigens
- promote the killing and/or removal of the immune complex formed through the activation of effector mechanisms
what determines the antibody classes and sub classes
defined by the heavy chain constant region
Describe IgM
(accounts for about 10% of the serum antibody pool)
Heavy chain encoded by Mu gene with 4 CH domains
Major antibody in primary response
Monomer forms the BCR on most B cells in association with Iga and Igb chains
Pentameric; stabilised by J-chain
Low affinity but high avidity
Activates complement (Classical)
advantage of IgM pentameric structure
it provides 10 identical binding sites, which can dramatically increase the avidity with which IgM binds its cognate antigen. Given that serum IgM commonly functions to eliminate bacteria containing low affinity, polysaccharide antigens, the increased avidity provided by the pentameric structure provides an important functional advantage.
Describe IgG
(the predominant antibody isotype in normal human serum)
Monomeric, heavy chain g gene, 3 CH domains
Major circulating Ig (70-75%)
Major antibody in secondary response
4 sub-classes G1; G2; G3; G4
Can cross placenta and protect foetus
Activates complement (Classical) and Opsonin (FcR)
Describe IgA
(the predominant antibody isotype present in seromucous secretions)
Heavy chain encoded by a-gene, 3 CH domains
15-20% circulating Ig
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
Does not activate complement (by classical pathway)
How do IgA subclasses differ?
The IgA1 and IgA2 subclasses differ substantially in the structure of their hinge regions:
* the hinge of IgA1 is extended and bears O-linked
oligosaccharides;
* the hinge of IgA2 is truncated, relative to IgA1.
A deficit in the addition of O-linked sugars within the hinge region of IgA1 protein has been linked with the disease IgA nephropathy.
Describe IgA secretion
IgA dimers secreted into the intestinal lamina propria by
plasma cells bind to poly-Ig receptors on the internal (basolateral) surface of the epithelial cells.
The sIgA–receptor complex is then endocytosed and transported across the cell while still bound to the membrane of transport vesicles.
These vesicles fuse with the plasma membrane at the luminal surface, releasing IgA dimers with bound secretory component derived from cleavage of the receptor.
The dimeric IgA is protected from proteolytic enzymes in the lumen by the presence of this secretory component.
The released secretory form of IgA is relatively
resistant to cleavage by enzymes in the gut and is
comprised of:
* two units of IgA;
* J chain; and
* a secretory component (mass 70 kDa)
Describe outcome of BCR Ligation
Clonal expansion: one B cell when activated becomes a clone of cells each with exactly the same BCR specificity
The majority of cells become effector/plasma cells which produce antibody
Plasma cells have a limited life-span and apoptose after a few days
A small proportion of cells remain as long-lived memory cells
Each clone secretes only one type of antibody, with a unique specificity.
The secreted antibody has the same binding specificity as the original B cell receptor
Describe Surface bound Ig (BCR)
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
Some tissue bias, e.g. mucosal B cells express IgA
Surface bound Ig has short intracytoplasmic tail and is associated with ‘accessory’ molecules to form the BCR
Iga (CD79a) and Igb (CD79b)
Ligation of the BCR results in the phosphorylation of the ITAM molecules (Immunoreceptor Tyrosine- based Activation Motifs) Leads to downstream cascade of events resulting in differentiation into plasma cell and antibody production
Describe Monoclonal Antibodies Production
- Mouse infected with specific antigen - this induces production of antibodies against antigen
- Spleen removed and homogenized into cell suspension (B cells that produce antibodies against injected antigen)
- Cells from suspension are then mixed with myeloma cells
- These cells wold fuse and make hybridomas (hybrid cells)
