Antibody interaction with antigen Flashcards
What is the antigen?
Ab recognise conformational antigens-composed of several sequentially discontinuous segments brought together by the folding of the molecule.
epitope - part of antigen recognised by antibody
compliementary part of ab is the paratope
paratope and epitope are conformationally complimentary.
Antibody structure
Hypervariable region
CDR loops interact with antigen
Poke out the B-sheets of the Ig domain so they have access to the antigen
Three regions of variability – CDR1,2,3 – hypervariable regions on the molecule, bind to antigen by epitope
Binding of antibody to antigen
Neither the antibody nor the antigen is changed by binding
One antibody binding site binds to one epitope on the antigen
Binding is non-covalent and is reversible
Non-covalent interaction due to:
- Ionic bonds
- H-bonds
- Hydrophobic bonds
- Van der Waals forces
small forces add up to a strong interaction, all dependent on the distance between the antigen and antibody binding parts.
What is anitbody affinity measured by?
Measured by law of mass action.
What is antibody avidity?
When you have a number of binding sites on antibody molecule that can bind a number of antigen epitopes.
Monovalent if you have sparsely available binding sites, only one arm of antibody can bind to it. - low avidity
Bivalent – two arms can bind to it – high avidity
Polyvalent – lots of binding sites interacting with epitope, strong binding – very high avidity
What additional signals are needed for antibody production?
Additional signals are needed by the B cell once the BCR is ligated and Igalpha and Igbeta signal sent.
T cell help: cytokines – support B cells to make antibodies , cytokine environment controls class switching from IgM
Additional co-stimulation e.g. CD19 – cluster of differentiation
Activated B cells induces proliferation 1000-10000x
Differentiation into plasma cell, some produce memory cells
Secondary antibody response bigger and gives you more memory cells.
How do antibodies neutralise virus and toxins?
May inhibit virus-cell interaction, prevent endocytosis of virus or prevent uncoating inside endosome. More effective with complement.
Abs bind bacterial exotoxins: antibodies neutralise their effect by preventing attachment to cellular receptors (e.g. binding of cholera toxin to ganglioside GM1)
Stimulate toxin clearance from body (Fc-receptor mediated)
IgG and IgA are important neutralising antibodies
What is opsonisation?
The coating (binding) of particles by either Ab, Complement or APP (eg CRP)
Ab bind microorganisms via the Fab and to cells by the Fc receptor
Opsonisation increases the efficiency of the phagocytic process, allowing the organism to be cleared more effectively
What are the functions of complement?
Antibody independent ‘innate’ immunity
Classical and alternate pathways
Functions
- Chemotaxis – recruiting cells to certain area
- Opsonisation
- Lysis of target cells
- Priming of the adaptive immune response
What are FC receptors involved in?
How antibodies interact with cells, some have different structures
- associate with y-chain
- Activation occurs due to aggregation of receptors and signalling via immunoreceptor tyrosine-based activation or inhibition motifs /ITAM ITIM
Causes ADCC, phagocytosis, apoptosis, mediator release and can enhance antigen presentation
What does FcyR1 (CD64) bind to and what is it involved in?
Binds monomeric IgG1 and IgG3 with high affinity and IgG4 with low affinity
No binding to IgG2
Expressed on mononuclear phagocytes e.g macrophages
Involved in phagocytosis of immune complexes and mediator release
3 extracellular Ig domains
Associated with y-chain ITAM
What are the two forms of FcyRII (CD32) involved in?
FcyRIIa
- Wide cellular distribution
- Moderate affinity for monomeric IgG1 and IgG3
- High affinity for complexed IgG
- Has ITAM
FcyRIIb
- Same specificity for Ig but has ITIM – may be to do with shutting down a B cell response
- ITIM is a negative signal, blocks b cell receptor signalling, stops antibodies being produced further, negative feedback on the process
inhibition of antibody response
What are the two forms of FcyrIII (CD16)
FcyRIIIa:
- Transmembrane molecule with moderate affinity for monomeric IgG
- Associated with y-, B-, z- h- chains of the CD3 complex as ITAMs
- Expressed on monocytes, macrophages, NK cells and some T cells.
- ADCC
FcyRIIIb:
- GPI linked with low affinity for monomeric IgG
- Expressed on granulocytes - neutrophils and basophils
- Activates by lipid raft formation and associates – bringing together molecules on cell surface
What is FcaRI (CD89) associated with?
Associated with y-chain
Expressed on myeloid cells
Can trigger phagocytosis, cell lysis and the release of inflammatory mediators
Binds both IgA1 and IgA2
What is FceRI associated with?
Very high affinity receptor for IgE (10^10L/mol)
Associates with y-chains and b-chain (aBy2 receptor unit) - strong signalling complex
FceRI expressed on mast cells and basophils
Receptors always saturated (hencelow serum IgE) - gives quick activation
X-linking of these Ab molecules bound to FceRI leads to mediator release e.g histamine
