Antigen-Antibody Interactions Flashcards
Whats the difference between the epitope and the paratope?
The epitope is the part of the antigen which is recognised by the antibody also known as there antigenic determinant
The paratope is the antigen binding site on the antibody. it is comprised of 17 amino acids of the antibody and 16 residues of the lysozyme molecule. It has 6 CDRs (Complimentary Determining Regions), 3 on heavy chain and 3 on light chain. This is where the epitope binds
The epitope and the paratope are conformationally complimentary
What determines the strength of the Ab-Ag interaction?
Antibody-Antigen interactions form weak non-covelant bonds such as
- H-bonds -> -OH δ+ —— δ- NH2- (usually between water molecules)
- ionic bonds/ electrostatic -> -NH3 + —— - OOC-
- Hydrophobic bonds -> With tyr, phe, leu, ile etc – allow amino acids that don’t like water to get together
- Van der Waals forces -> between transient dipoles (from random fluctuations in electron distribution in molecules) (short range)
These bonds combined can exert a large force but there needs to be a very short distance between Ab and Ag for the bond to form
Define Affinity
The strength of the force which can be measured is the affinity. This is also defined as the equilibrium constant (K) which can be measured by;
Ka = [AbAg]/[Ab] x [Ag] down to Law of Mass Action because the reaction is reversible .
Define Avidity
Avidity is the combination of affinities of binding between antigens and antibody binding sites and functionally avidity is likely to be more important than affinity.
1 antigen and 1 binding site = monovalent which equals a small avidity e.g. IgG (but has 2 binding sites)
- usually known as instrinsic avidity
multiple antigens and multiple binding sites e.g. IgM (10 potential sites) = Polyvalent = high avidity
- usually known as functional avidity
And subsequently the greater the avidity, the greater the equilibrium constant
Describe the functions of an antibody
NEUTRALISATION (virus)
- Some cells are better at neutralising viruses than others. The antibody works by binding to the docking site or the host entry site of the virus, preventing viruses from entering cells.
NEUTRALISATION (toxin)
• Abs bind bacterial exotoxins: neutralise their effect by preventing attachment to cellular receptors (e.g. binding of cholera toxin to ganglioside GM1) – then mopping them off
- preventing endocytosis or uncaring inside endoscope. We can also use CD4
• Stimulate toxin clearance from the body (Fc-receptor mediated )
OPSONISATION:
- The antibody will tag the surface of the bacteria, and recruit other cells (such as macrophages) towards it.
- The macrophage will have Fc receptors on it that is attracted towards the active site.
- The antibody will induce the macrophage to go through Antibody-Dependent Cellular Phagocytosis (ADCP).
- For tumour cells, the antibody will tag it, but since it’s too big to phagocytose, it will instead recruit NK cells to perform ADCC (Antibody-Dependent Cellular Cytotoxicity). -> NK cells produce chemicals make the tumour cell apoptose.
Antibodies can bind together to form complexes, and agglutinate when they bind to a pathogen, rendering it useless and allowing it to precipitate out of the blood.
COMPLEMENT FIXING:
- This is Antibody Independent “Innate” immunity as it doesn’t need Abs but works better with them.
- Antibodies form complexes with complement molecules, which can cause it to phagocytose.
- There are the Classical and Alternate pathways
- Can cause chemotaxis, opsonisation, lysis of target cells (MAC complex punching membrane holes) and Priming of the adaptive Immune Response.
What are Fc receptors (FCR) and what is their general function?
A Fc receptor is a protein associated with the gamma chain, found on the surface of certain cells – including B lymphocytes, follicular dendritic cells, natural killer cells, macrophages, neutrophils, eosinophils, basophils, human platelets, and mast cells – that contribute to the protective functions of the immune system.
Basically it is how the Antibody interacts with the cell so the cell can recognise that an antibody has bound a virus or antigen
Activation occurs due to aggregation of receptors –common in immune system
Some receptors have ITAMs/ITIMs (Immunoreceptor Tyrosine-based Activation/Inhibition Motifs)
Their functions include ADCC (Antibody-dependent cell-mediated cytotoxicity), phagocytosis, apoptosis, mediator release and enhancing antigen presentation
What is Fcg (gamma) R1?
FCgRI (CD64):
• Binds monomeric IgG1 and IgG3 with high affinity
• Binds IgG4 with low affinity
• No binding to IgG2
- Expressed on mononuclear phagocytes such as neutrophils or monocytes
- Involved in phagocytosis of immune complexes and causes mediator release
- 3 extracellular Ig light domains on surface
- Associated with gamma-chain ITAM motif – interacts through this to the cell
What is FcgR2a and FcgR2b
FcgRIIa
• Wide cellular distribution
• Moderate affinity for monomeric IgG1 and IgG3.
• High affinity for complexed IgG
• Explains how in some cells you can specific effects in special interactions
• Has its own ITAM – ligates on cell surface
FcgRIIb:
- Same specificity for Ig
- But it has an ITIM! – inhibitory version of ITAM which works by producing negative feedback on the process
and is Ab concentration dependent - it produces a negative signal on IgA and IgB to stop stimulation
What is FcgR3
FCgRIII (CD16):
• 2 forms
(A)
- a transmembrane molecule – expressed on NK cells and certain T cells
- Low affinity for monomeric IgG – associated with alpha, gamma and zeta chain on CD3
- Most likely to result in ADCC - Antibody-dependent cellular cytotoxicity
- Expressed on neutrophils and basophils
- Activates by lipid raft formation and associates with other membrane signalling molecules
(B)
- glycosyl phosphatidyl inositol (GPI) linked. – expressed on neutrophils
- Associated with g-chain
- Low affinity
- Expressed on myeloid cells
- Can trigger phagocytosis, cell lysis and the release of inflammatory mediators
- Binds both IgA1 and IgA2
What is FcER and its 2 different forms?
FcER - the Fc receptor for IgE molecules and involved heavily in allergy response. Has 2 forms and are expressed on mast cells
FcERI
• Very high affinity for IgE (1010L/mol) – Anaphylactic response – quick reaction because you have already produce the Abs
• Receptors always saturated (low serum IgE)
• X-linking of these Ab molecules bound to FceRI leads to mediator release eg Histamine
FcERII (CD23)
• aka - CD23a and CD23b
• CD23a: expressed on B cells and involved in IgE production
• CD23b expressed on lots of cell types and induced by IL-4
