Organ transplantation is In many cases the preferred treatment for the
chronic failure of the major organs. There has been considerable succ
ess in preventing the rejection of transplanted organs, and further Im
provements In the outcome of transplantation are in prospect. However,
the main limitation on the fullest possible use of organ transplantat
ion is the shortage of donated human organs. One solution to this prob
lem would be xenotransplantation, the use of animals Instead of humans
as the source of organs and tissues for transplant. The Immunological
barriers to this procedure are now well defined and this review descr
ibes the various strategies that are being developed to try to overcom
e them.