Transplantation

TRANSPLANTATION:
An autograft (transfer of an individual s own tissue to another site in the
body) is always permanently accepted, i.e., it always "takes."
A syngeneic graft is a transfer of tissue between genetically identical
individuals, i.e., identical twins, and almost always "takes" permanently.
A xenograft, a transfer of tissue between different species, is always
rejected by an immunocompetent recipient.
An allograft is a graft between genetically different members of the same
species, e.g., from one human to another. Allografts are usually rejected unless
the recipient is given immunosuppressive drugs. The severity and rapidity of
the rejection will vary depending on the degree of the differences between the
donor and the recipient at the MHC loci.
Allograft Rejection: Unless immunosuppressive measures are taken, allografts are
rejected by a process called the allograft reaction. In an acute allograft reaction,
vascularization of the graft is normal initially, but in 11 to 14 days, marked reduction
in circulation and mononuclear cell infiltration occurs, with eventual necrosis. This is
called a primary (first-set) reaction which mediated by T-lymphocytes sensitized to
class I and class II antigens of the allograft, lymphokines, activated monocytes and
macrophages.