The immune system defends the body against disease. Lymphocytes are one type of white blood cell that helps the body resist infection by recognizing harmful foreign substances, such as bacteria, and eliminating them. The ability of the vertebrate immune system to distinguish self from nonself depends largely on a group of protein makers (antigens) known as the major histocompatibility complex (MHC). These markers are present on the surface of every cell are slightly different in different individuals. The genes that code for these proteins are found linked together on one chromosone (chromosome 6 in humans).

In humans the MHC is called the HLA (human leukocyte antigen) group. HLA is determined by five different linked genes. These genes are all polymorphic - that is, within the population there are multiple alleles at each locus. Tissues from the same individual or from identical twins have the same HLA alleles and thus the same HLA antigens.

Skin can be successfully transplanted from one part of the body to another. However, when skin is taken from one indiviual and grafted onto the body of another, the skin graft is rejected and it sloughs off. Recall that tissue from the same indiviual or from identical twins have the same HLA alleles and thus the same HLA antigens. Such tissue are compatible. Because its HLA antigens are the same, the tissue is not rejected. Tissue transplanted from one location to the other in the same indiviual is called an autograft.

Because of the polymorphism (multiple alleles) of the HLA genes, it is difficult to find identical matches among strangers. If a tissue or organ is taken from a donor and transplanted to the body of a unrelated host, several of the HLA antigens are likely to be different. Such a graft made between the members of the same species but of different genetic makeup is called a homograft. The host's immune system regards the graft as foreign and launches an effective immune response called a graft rejection. T lymphocytes attack the transplanted tissue and can destroy it within a few days. Before transplants are performed, tissues from the patient and from potential donors must be typed and matched as well as possible.

Received from Villee, C.A., Soloman, E.P., Martin, C.E., Martin, D.W., Berg, L.R., Davis, P.W., "Biology second edition", Saunders College Publishing, 1989, 1985 p988, p1002