The transplant rejection is condition in which cells from the transplanted tissue of a donor initiate an immunologic attack on the cells and tissue of the recipient. The donated organ, called an allograft if the donor and recipient are unrelated, is transplanted into the recipient, called the host. Rejection can occur when the host?s immune responses are directed against the graft. The rapidity and reversibility of rejection depend on the various mechanisms involved, such as peritransplant ischemia and mechanical trauma, preformed antibody interactions with graft antigens, alloantigen-reactive T cells, and abnormal tissue remodeling. Graft rejection syndromes can be divided into three subtypes, based on timing of onset and mechanisms involved.