1 At which level DNA or RNA are each of the following activi

1. At which level, DNA or RNA are each of the following activities determined?

a. Membrane vs secreted antibody

b. IgM vs IgD

c. V-J joining

d. IgA vs IgE

e. V-D-J joining

2. Describe the role of AID in antibody production. What would be the consequence of an inactivating mutation in this enzyme?

3. Explain why despite of the fact that each mammalian cell has two alleles for each gene locus, a B cell can only produce antibodies of single antigen specificity.

Solution

1. ANS:

RNA activities level is determined at:

a. Membrane vs secreted Ab
b. IgM vs IgD

DNA activities level is determined at

c. V-J joining

d. IgA vs IgE

e. V-D-J joining

2. ANS:

AID role in antibody production:

It plays a very important role in somatic hyper mutation in antibody production.

Somatic hyper mutation occurs only after an antigen activates a matured B-cell present in the germinal center. The mutation rate is very high 10.

The enzyme that induces mutation is Activation induced deaminase (AID).

It deaminates cytosine to uracil (Thymidine) and a change in base pairing from GC to AT occurs and it results in affinity in maturation, the B cell with high affinity receptor are selected and can produced high affinity antibodies and B-cell with low affinity receptor dies.

3. ANS:

Mono specificity of a B-cell due to the allelic exclusion, B cell can only produce antibodies of single antigen specificity.

Single antigen specificity (immune specificity) is regulated by receptor selection, by clonal selection.

In clonal selection the immature B cell undergoes negative clonal selection and acquire the ability to differenceate self-antigen and a foreign antigen (non self).

In negative selection those B-cells that do not interact with self-antigen are selected if it interacts with self-antigen clonal deletion occurs and the B-cell will die.

This clonal selection mechanism plays an important role in allelic exclusion and diversification of the antigen specificity.