Scarcely anyone would dispute that donor-specific B cells and the Abs that they produce can cause rejection of transplants. Less clear and more controversial, however, is the possibility that donor-specific B cells and the Abs that they produce are one or more means by which transplants can be protected from injury. In this article, we review and discuss this possibility and consider how less well-known functions of B cells and Abs might impact on the design of therapeutics and the management of transplant recipients.