Xenoreactive natural antibodies

Shortages of human organs for transplantation have made it necessary to exa mine the possibility of using nonhuman organs for xenotransplantation-the t ransplantation of tissues between different species. Pigs are now regarded as the most likely species to serve as donors for clinical xenotransplantat ion. However, rejection of pig tissues and organs, mediated by the host's i mmune system, remains a major barrier to successful xenotransplantation. Th e primary immunological hurdle to overcome is rejection mediated by antibod ies in the host that recognize antigens present on xenogeneic tissues. Sinc e these antibodies are produced naturally in the host without immunization, they are termed natural antibodies. Here, we review the nature of xenoreac tive natural antibodies directed toward pig tissues, and summarize recent p rogress in the field of xenotransplantation directed at overcoming humoral rejection of porcine xenografts.