Transplantation is an act of transferring cells, tissues, or organ from one site to other. Graft is implanted cell, tissue or organ. Development of the field of organ and tissue transplantation has accelerated remarkably since the human major histocompatibility complex (MHC) was discovered in 1967. Matching of donor and recipient for MHC antigens has been shown to have a significant positive effect on graft acceptance. The roles of the different components of the immune system involved in the tolerance or rejection of grafts and in graft-versus-host disease have been clarified. These components include: antibodies, antigen presenting cells, helper and cytotoxic t cell subsets, immune cell surface molecules, signaling mechanisms and cytokines that they release. The development of pharmacologic and biological agents that interfere with the all immune response and graft rejection has had a crucial role in the success of organ transplantation. Combinations of these agents work synergistically, leading to lower doses of immunosuppressive drugs and reduced toxicity. Significant numbers of successful solid organ transplants include those of the kidneys, liver, heart and lung.