22. The number of MHC protein combinations possible in a given population is enormous. However, an individual in thatpopulation has only a couple of MHC possibilities. Why?
(A) Once a B cell has matured in the bone marrow, it is limited to two MHC response categories.
(B) Once a T cell has matured in the thymus, it can only respond to two MHC categories.
(C) MHC proteins from one individual can only be of class I or class II.
(D) The MHC proteins are made from several different gene regions that are capable of rearranging in a number of ways.
(E) Each of the MHC genes has a large number of alleles, but each individual only inherits 2 for each gene.