B Cells

Question 1

Describe antibodies

Answer 1

• Dimers bound together by disulphide bond
• Each dimer is same and has heavy and light chain (heavy in membrane)
• Each dimer has constant and variable regions (VL and VH)
• Fc region is constant region/stalk
• Variable region- made up of three loops from heavy chain and three from light- these are complementarity determining regions CDRs- 6 loops form surface that determines complementarity

Question 2

Describe antibody-antigen binding

Answer 2

• No covalent bonds involved, must get close to interact (Van de Waals)
• Generally bind around a 6 amino acid area
• Antigen-antibody reactions are reversible
• Binding reaches an equilibrium- equilibrium constant/affinity K (generally around 10^7-10^8 M^-1 which is very high)
• DNA as antigen- charged forces rather than VdW, lies along top
• Sugar as antigen- hydrophobic and hydrogen bonds, lies along top as don’t bend well
• Small molecules as antigen (penicillin)- penetrate between CDR loops

Question 3

Outline the generation of diversity

Answer 3

• Occurs during B cell development in bone marrow
• Each B cell has only one specificity
• Wasteful process- ~90% cells die in process
• Generates billions of possible sequences from only 200 genes

Question 4

Outline VDJ recombination

Answer 4

• Locus for IgH is divided into mini-genes which each contribute to one part of antibody
• Variable(V)~40, Diversity(D)~30, Joining(J)~6
• B cell IgH DNA random selection of one V, D and J and ligation together-> Somatic rearrangement
• VDJ then coded for entire variable region
• Only one allele is used, other switches off so cell only makes one type of antibody

Question 5

Where are Ig genes located?

Answer 5

• IgH- chromosome 14
• IgL can be:
  Igλ- chromosome 22 or
  Igκ- chromosome 2

Question 6

How is VDJ recombination mediated?

Answer 6

• Recombinase enzymes (encoded by recombination activating genes RAG-1&2)
• Recognise recombination signal sequence flanking V, D and J gene segments
• Joining is imprecise- recombinatorial inaccuracies increase diversity
- deletion of bps before joining-> new sequences (multiple of 3)
- addition of bps between joins (some by Terminal deoxynucleotidyl Transferase TdT

Question 7

What are the mechanisms of antibody diversity?

Answer 7

• Multiple VDJ minigenes
• Imprecise joining
• Independent light and heavy chains
• Somatic hypermutation (when stimulated by antigen- those with slightly better complementarity are selected for)