Adaptive immunity

Question 1

What are the forms of Ig?

Answer 1

i) Surface Ig - embedded in the B cell membrane - is the B cell’s antigen receptor
ii) Secreted/soluble Ig - secreted by B cells after they bind their antigen, proliferate and differentiate - controls extracellular microbes, and mediates effector functions

Question 2

How is antibody diversity created?

Answer 2

Rearrangement are the result of random somatic gene rearrangements (D-J, V-DJ, V-J),

Junctional diversity - makes the greatest contribution to overall diversity, alter reading frame of protein because different segments are different sizes

Somatic hypermutation - B cells, after matured can further alter antibody and can enhance binding to antibody e.g. isotype switching

Question 3

What are the checkpoints for antibody gene rearrangement?

Answer 3

Checkpoints:
heavy chain - signal transduction @ the Large pre-B cell stage

light chain - IgM expressed @ immature B cell stage

final checkpoint - If a B cell is found to be reactive to self it is deleted

Question 4

What is affinity maturation?

Answer 4

As antigen levels decrease during an immune response, B cells with mutations that result in high affinity surface Ig are preferentially selected for survival (clonal selection!)

Question 5

What are the differences between antibodies and TCRs?

Answer 5

• TCRs are only membrane bound (don’t secrete)

- TCRs don’t undergo somatic hypermutation (don’t want auto-reactivity)

Question 6

What are the differences between MHC class I and MHC class II molecules?

Answer 6

MHC class I 
- sample antigens from cytosol (e.g. virus, tumor or self antigens)
- expressed on all nucleated cells
-binds shorter peptides (8-11 aa)
-express HLA A, B and C
MHC class II 
- sample antigens from endosomes, derived from extracellularly ingested material (e.g. bacterial antigens)
-expressed on APCs
-binds longer peptides (10-30 aa)
-express HLA DP, DQ, DR

Question 7

How is diversity of antigen presentation achieved?

Answer 7

1. Polygenic - e.g. HLA A, B, C in MHC class I and HLA DP, DQ, DR in MHC class II
2. Co-dominant gene expression - alleles expressed from both parents are expressed equally e.g. HLA A, B and C from mother and father both expressed, so 6 MHC molecules are expressed
3. Polymorphisms - large number of alleles for MHC genes e.g. HLA A2, A3, A4 etc.