Schematic of proposed enhanced alloimmunization. Response to infection. Peptides containing a polymorphism (designated by A) from a microbe will be processed and presented by host antigen-presenting cells; peptides presented in MHC II will be recognized by CD4⁺ T cells. However, in the absence of a B-cell epitope, the B cells will not be able to receive CD4⁺ T-cell help to generate an antibody response. Response to transfusion. Upon a second antigenic exposure (transfusion) that contains the same polymorphism A, the polymorphism in the blood group constitutes not only a CD4⁺ T-cell epitope but also a de novo B-cell epitope. Through receptor-mediated endocytosis, naive B cells phagocytose the polymorphism-containing blood group molecule. The polymorphism is then presented on the MHC II of B cells to the preformed helper or memory CD4⁺ T cells generated against the microbial infection. The B cells are then stimulated to differentiate into plasma cells that secrete antibodies against the polymorphism.