It is becoming increasingly recognised in transplant patients that the development of antibodies against the transplant plays a significant role in transplant rejection and we are trying to identify the pathways that are responsible for this. Antibodies are produced following activation of B lymphocytes, and this requires the help of T cells as well; antibodies are also produced following reactivation of memory B cells, which are less dependent on T cell help. We are studying the development of B cell memory and we are working on approaches for monitoring memory B cells in patients who have lost a previous transplant or who have increased levels of anti-HLA antibodies.