Fig 2.

Class II-restricted antigen presentation by alloreactive B cells to CD4⁺ T cells with indirect specificity results in their cognate interaction with T follicular helper cells (Tfh) at the T:B interface and in the germinal center, leading to the elaboration of DSA responses. It is possible that B cells also engage in cognate interactions with non-Tfh T cells, including Th1, Th2, Th17 and Tregs, but where this occurs within the lymph node or spleen requires further clarification. The ability of B cells to cross-present exogenous donor antigens on MHC Class I has not been described, so B cells most likely cannot engage in cognate interactions with alloreactive CD8⁺ T cells that have indirect specificity.