The major histocompatibility gene complex (MHC) is a group of genes found in all vertebrates that play a major role in the immune system. Its function is to provide the immune system with information about the type and health of cells in the body, and to help distinguish between self and non-self cells. There are several synonyms for this complex, including the histocompatibility complex, the human leukocyte antigen (HLA) complex, and the major histocompatibility complex. These terms all refer to the same group of genes that are essential for immune system function. Understanding the MHC is crucial for researchers studying immune-related diseases, transplant compatibility, and vaccine development.