Antibodies

What do antibodies recognise?

proteins, carbs, nucleic acids, glycolipids and small inorganic molecules

antibody responses

• interacts with complement for cell lysis

• interact with phagocytes via Fc region

• neutralising effects

IgM - pentamer

1st antibody produced, involved in exchange of antibodies between mother and foetus

IgG - 4 different subtypes

most abundant, activates classical complement, mediates ADCC by NK cells and causes opsonisation

IgD

uncertain role

IgE

works against parasites, involved in type I hypersensitivity reactions (allergy)

IgA

dimer which activates complement pathway, adheres microbes to mucosal surfaces and involved in inflammation

antibody structure

• 2 identical light chains (25 kDa)

• 2 identical heavy chains (50 kDa)

• Fab region of both heavy and light chains (variable region)

• Fc = heavy chain constant region

antibody flexibility

important for binding pathogens. Hinge allows some independent movement of Fab arms eg waving, rotating, Fc tail ‘wagging’ or Fab elbow bending

antigen-antibody complexes

formed by crosslinking of bound antigens and antigen binding sites to create molecule more identifiable to phagocytes

complementarity-determining regions (CDRs)

hypervariable (HV) regions

framework regions

regions between CDRs

paratope

CDRs which make up antibody combining site

epitope

antigen combining site which the paratope is complementary to

What is the most hypervariable region in the VH and VL domains of antibodies

HV3

How many hypervariable regions does each antibody have?

6 - The VH and VL regions each have HV1, HV2 and HV3

antibody domain structure

• barrel shaped

• antiparallel beta strands forming beta sheets

• beta sheets held together by disulphide bond

What happens to the hypervariable regions when the VH and VL domains pair?

the 3 HVs from each domain come together to form a single HV site at the tip of each arm - antigen binding site

How are antigen-antibody complexes held together?

electrostatic interactions, hydrogen bonds, Van der Waals, hydrophobic interactions and cation-pi interactions

electrostatic interactions

occur between charged side chains, with at least one being found in most antibody-antigen interactions

antibody repertoire

total antibody specificities available to an individual. At least 10^11 in humans

gene segments encoding heavy and light chains

organised into three genetic loci - kappa, lambda and heavy chain loci

V(D)J recombination - developing B cells

Heavy chains undergo V D and J recombination and light chains recombine V and J segments. Mediated by RAG 1/ 2 enzymes

junctional diversity - occurs alongside V(D)J recombination

RAG 1/2 and TdT add or remove nucleotides between V, D and J altering recombination sites

junctional diversity step by step

• RAG complex cleaves recombination signal sequences → DNA hairpin

• hairpin cleavage by RAG → palindromic P-nucleotides

• TdT adds N-nucleotides

• strands pair

• unpaired nucleotides removed by exonuclease

• gaps filled by DNA synthesis and ligation

somatic hypermutation

rapid introduction of point mutations, especially in HVRs, after B cell encounters antigen creating B cells with higher affinity

class switch recombination (IgM → )

B cells change constant region of heavy chain after activation so that antigen specificity can be linked with different functions

how do antibodies activate complement?

C1q activated by Fc of antibody bound to antigen. C1r&s associate → C1 complex which is cleaved→ C2 and C4 which form C3 convertase forming membrane attack complex