Major histocompatibility complex (MHC) molecules are membrane-bound proteins that serve as signal beacons to the immune system. Some of these molecules bind peptide fragments derived from pathogens, displaying the fragments on the cell surface for recognition by T cells, while others present these fragments directly to the immune system.
There are two types of MHC molecules: MHC I and MHC II. MHC I molecules are present in almost all body tissues. MHC II molecules are present exclusively on antigen-presenting cells, such as macrophages, B cells and dendritic cells. The immune system constantly monitors MHC I molecules, and when it detects a foreign protein from an intracellular parasite, it destroys the cell to protect the body as a whole.