Mature B cells undergo somatic hypermutation after activation, which, after affinity maturation, results in the production of antibody with a higher affinity for antigen than in the primary antibody response

Question

Suggest some reasons why T cells have not evolved the same capacity.

What will be an ideal response?

Answer 1

Because T cells drive almost all immune responses, once they have been activated their receptors must continue to recognize the exact complex of foreign antigen and MHC molecule (which does not itself change) that activated them. Because of this requirement for dual recognition (MHC restriction), somatic hypermutation would be more likely than not to change the T-cell receptor to make it unable to recognize either the peptide or the MHC molecule, or the combination of both, thus rendering it unable to give help to B cells or to attack infected cells. This would destroy both the primary immune response and the development of immunity. Even changes that simply increased the affinity of the T cell for its antigen would have no real advantage, because it would not make the immune response any stronger or improve immunological memory in the same way that affinity maturation of B cells does. Also, if somatic hypermutation changed the specificity of the T-cell receptor so that it now recognized a self peptide, this could result in an autoimmune reaction. These considerations do not apply to B cells, because they require T-cell help to produce antibody and will only receive it if their B-cell receptor still recognizes the original antigen.