Major histocompatibility complex
MHC molecules affect the way immune cells interact with other cells. Their function is to sense foreign antigens, and hence the presence of 'foreign' proteins. Adaptive immunity depends on this reaction.
Immune cells use MHC to communicate with each other on how to attack an infection, like a bacterium or virus. They do this by presenting information on the MHC and physically touching another cell. An autoimmune disease occurs when the immune cells do not recognize the MHC molecules of other cells and starts attacking its own body. Transplants are limited to those which get the least reaction from the MHC system in tests of tissue matches.