The unusual intensity of alloimmune responses reflects which of the following characteristics?

B. Recognition of the native structure of allogeneic MHC molecules.

C. The high frequency of T cells able to recognize directly allogeneic MHC antigens.

D. Stimulation of many T-cell receptors during the interaction of a T cell with an antigen-presenting cell.

E. The high frequency of antigen-presenting cells able to be recognized by T cells.

DISCUSSION: Allotransplantation evokes an unusually intense and rapid cellular immune response. In contrast to conventional cellular immune responses, in which foreign antigens are recognized only as peptides in the groove of self MHC antigens, allogeneic MHC antigens are recognized directly as native proteins on the surface of allogeneic antigenpresenting cells. Thus, a large fraction of antigen-presenting cells is able to present alloantigen in this fashion, and a large fraction (up to 10%) of T cells is able to respond.