Composite tissue allograft extends a helping hand to transplant immunologists.

The first successful human hand transplantation, performed on September 1998, has translated the scope of 'composite tissue allotransplantation' from research concepts into clinical practice. Beyond microsurgical problems that have been overcome several years ago, the main obstacle that still prevents the generalization of composite tissue allotransplantation is immunologic. This review, which summarizes the evidence obtained both from experimental animal models and from the first recipients of a hand transplant, is focused on the two immunological characteristics of composite allografts that set them apart from other solid organ allografts: (i) they contain skin tissue that elicits a strong immune response; and (ii) they contain lymphoid tissues (such as bone marrow and lymph nodes) that have the potential both to attack the recipient, and also to down-modulate the host immune response and induce tolerance. While on one hand, the composite tissue allografts raise new challenges to transplant immunologists, on the other they provide answers to questions that have remained unresolved for a long time. In this sense, composite tissue allografts extend a helping hand to transplant immunologists.