The potential for B cells to play a role in transplantation has been recently increasingly recognized. Given that the field has focused almost exclusively on the T cell, here we review B-cell biology. We first describe the B-cell response to protein antigen, and then examine the antibody-independent effector functions of B cells, in particular antigen presentation to T cells. Finally we use autoimmunity as an analogy to focus on the potential role for B cells in transplantation. The direct evidence for this is then marshalled in the accompanying article by Zarkhin and Sarwal.