Fig. 2.

B cells with specificity for allogeneic HLA must first be activated by the combination of antigen and T cell help. Viral-specific CD4 T cells may cross-react against allogeneic HLA-derived peptides presented on autologous HLA molecules (indirect allorecognition) by recipient APCs (Dendritic cells, B cells, or activated T cells). In the germinal center reaction, such B cells may undergo somatic hypermutation with the development of high-affinity surface IgG with HLA specificity. Progeny B cells would leave the germinal center and differentiate into either memory B cells or antibody-producing plasma cells. APCs antigen presenting cells