A syngeneic graft is the transfer of tissue between genetically identical individuals

(identical twins). This type of graft is usually successful.

An allograft, or homograft, is a graft between genetically different members of the same species.

This type of graft would be between two different humans and would most likely be rejected unless the recipient

is given immunosuppressive drugs.

An autograft is a transfer of an individual's own tissue to his or her own body and is nearly always

successful.

A xenograft (formerly heterograft) is a transfer of tissue between different species and is always rejected except for a few exceptions (e.g., pig heart valve).

Graft Rejection

Hyperacute rejection occurs within minutes to a few hours of the time of transplantation,

due to the destruction of the transplanted tissue by preformed antibodies reacting with antigens found on the transplanted tissue that activate complement and destroy the target tissue.

Preformed antibodies can also be due to presensitization to a previous graft, blood transfusion, or pregnancy.

Acute rejection is due to type II and type IV reactions.

Chronic rejection, due to the presence of cell-mediated immunity to minor HLA antigens occurs in

allograft transplantation months to even years after the transplant. Chronic rejection is generally caused by

both humoral and cell-mediated immunity.

An accelerated acute rejection, occurring in 3-5 days, can be caused by tissue infiltration and destruction by presensitized T lymphocytes and macrophages and/or antibody-dependent, cell-mediated cytotoxicity (ADCC). Note that this is not a hyperacute reaction.