Histocompatibility refers to the means by which a eukaryotic cell can be identified. The phenomenon is the result of the presence of proteins on the surface of cells. These proteins are referred to as histocompatibility molecules. The histocompatibility molecules on the cells of one individual of a species are unique. Thus, if the cell is transplanted into another person, the cell will be recognized by the immune system as being foreign. The histocompatibility molecules act as an antigen in the recipient, and so can also be called a histocompatibility antigen or transplantation antigen. This is the basis of the rejection of transplanted material.
Identical twins have the same histocompatibility molecules on their cells. Thus, tissue can be successfully transplanted from one individual to the other, because the tissue will essentially not be foreign. However, for unrelated individuals, cells will have their own signature chemistry with respect to the histocompatibility molecules. Tissue from one individual will be recognized as foreign in another individual.
The suite of histocompatibility molecules present on the surface of a cell is also referred to as the histocompatibility complex. There are two classes of these molecules. The first class is called class I molecules. These molecules are made up of a portion that is embedded in the cell membrane and a portion that protrudes out from the membrane's outer surface. The protruding portion is composed of both protein and sugar (carbohydrate). Some of the human leukocyte antigens are examples of class I molecules.
The class I molecules function to chemically tag a cell so that the cell will be recognized and categorized by the T lymphocyte cells of the immune system. The T cell will recognize a region of the histocompatibility complex as "self." Because of this recognition, there will not be an immune response initiated against the cell. But, in another host, where the same region is chemically different from class I groups on the host cells, the introduced cells would be recognized as foreign by the T lymphocytes.
Another class of histocompatibility molecules called class II are anchored into the cell membrane by have two segments of the molecule. At the outer surface of the cell the molecule contains an antigen that has been acquired from the surrounding environment when particles were taken in and degraded by host processes. This is called antigen presentation.
Class II molecules are on the surface of macrophages and B-lymphocytes. These immune cells function to process cells and present the antigens from these cells to T lymphocytes. This is done to increase the repertoire of antibodies that an organism possesses. Antigen presentation of histocompatibility molecules "primes" the immune system. When an invading organism is detected, the immune response can occur much more swiftly than if no exposure to the antigen had ever occurred.
The role of the histocompatibility complexes in immune recognition of "self " and "non-self" is the reason why transplants are typically accompanied by the administration of drugs that dampen down the immune response of the host. Only by nullifying the host recognition of the class I and class II histocompatibility complexes can the transplant be maintained.
The genes that encode the histocompatibility determinants are clustered together on the chromosome. These clusters are referred to as the major and minor histocompatibility complexes. The major compatibility genes are clustered together on one chromosome. The minor compatibility genes are located in several clusters throughout the genome.
Studies on mice, which also possess the histocompatibility complexes, have demonstrated that these complexes not only play a role in transplant rejection, but also function in the immune response to a variety of diseases. Mice that are genetically different for a given histocompatibility complex will respond differently to the same antigen. If a "non-self" histocompatibility complex is poorly recognized by the host immune system, then an inadequate immune response will ensue. The result can be the establishment of an infection.
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