The Major Histocompatibility Complex (MHC) is a large locus on vertebrate DNA that contains a set of closely linked polymorphic genes that code for cell surface proteins essential for the adaptive immune system. MHC molecules mediate the interactions of leukocytes, also called white blood cells, with other leukocytes or with body cells. They determine donor compatibility for organ transplant, as well as ones susceptibility to autoimmune diseases. The MHC is highly polymorphic, and each locus has many alleles. There are two major types of MHC protein molecules: class I and class II. Class I MHC molecules span the membrane of almost every cell in an organism, while class II molecules are restricted to cells of the immune system called macrophages and lymphocytes. MHC molecules are important components of the immune system because they allow T lymphocytes to detect cells, such as macrophages, that have ingested infectious microorganisms. When a macrophage engulfs a microorganism, it partially digests it and displays peptide fragments of the microbe on its surface, bound to MHC molecules. The T lymphocyte recognizes the foreign fragment attached to the MHC molecule and binds to it, stimulating an immune response.
In summary, the MHC is a set of genes that code for cell surface proteins essential for the adaptive immune system. MHC molecules mediate the interactions of leukocytes with other leukocytes or with body cells, and determine donor compatibility for organ transplant, as well as ones susceptibility to autoimmune diseases. MHC molecules are important components of the immune system because they allow T lymphocytes to detect cells that have ingested infectious microorganisms.
