MHC

Major Histocompatibility Complex (MHC)

The Major Histocompatibility Complex (MHC) is a set of cell surface proteins essential for the acquired immune system to recognize foreign molecules in vertebrates, which in turn determines histocompatibility. The MHC molecules mediate interactions of leukocytes, also called white blood cells (WBCs), with other leukocytes or with body cells. The MHC determines donor compatibility in organ transplantation, as well as one's susceptibility to an autoimmune disease via cross-reacting immunization.

Structure and Function

MHC molecules are divided into two main classes: MHC class I and MHC class II.

• MHC class I molecules are found on almost every cell in the body and present peptides (protein fragments) from inside the cell to cytotoxic T cells.
• MHC class II molecules are found only on certain immune cells, such as macrophages, dendritic cells, and B cells. They present antigens from outside the cell to helper T cells.

Genetic Organization

The genes encoding MHC molecules are highly polymorphic, meaning they have many different alleles, which allows for a diverse range of antigen presentation. The MHC genes are located on chromosome 6 in humans and are divided into three regions: Class I, Class II, and Class III.

• The Class I region contains genes for MHC class I molecules.
• The Class II region contains genes for MHC class II molecules.
• The Class III region contains genes that encode components of the complement system and other immune-related proteins.

Role in Disease

MHC molecules play a crucial role in the immune system's ability to recognize and respond to pathogens. However, they are also involved in the development of autoimmune diseases, where the immune system mistakenly attacks the body's own cells. Certain MHC alleles are associated with a higher risk of developing specific autoimmune diseases, such as type 1 diabetes, rheumatoid arthritis, and multiple sclerosis.

Clinical Significance

The MHC is critical in organ transplantation, as mismatched MHC molecules can lead to transplant rejection. Matching donor and recipient MHC molecules as closely as possible is essential for the success of transplants. MHC typing is also used in bone marrow transplantation and in the treatment of certain cancers.

Related Pages

• Immune system
• Antigen
• T cell receptor
• Human leukocyte antigen
• Transplant rejection
• Autoimmune disease