A severe shortage of transplant donors has led to a markedly prolonged
waiting time for renal transplantation and has thus sparked interest
in the use of animals in lieu of humans as a source of kidneys for tra
nsplantation. Clinical application of xenotransplantation is limited i
n large part by the severe immunologic reaction of the recipient again
st the graft. This immunologic reaction is mediated initially by compo
nents of natural immunity, such as xenoreactive antibodies, complement
and natural killer cells, and later by elicited humoral and cellular
immune responses that act in concert to disrupt the function of the en
dothelial lining of blood vessels, The past few years have brought con
sider able progress in elucidating the molecular and cellular basis of
xenograft rejection and in developing strategies to overcome it.