Histocompatibility complex is a group of genes in vertebrates that encodes the molecules responsible for self-recognition. These molecules play a crucial role in maintaining immune system homeostasis and preventing autoimmunity. Synonyms for histocompatibility complex include major histocompatibility complex (MHC), leukocyte antigen complex (LAC), antigen-presenting complex, and transplantation antigen. The MHC is divided into two classes, MHC class I and MHC class II, based on their structure, function, and expression on different cell types. These molecules present peptide antigens to T cells to initiate an immune response. The LAC, on the other hand, contains genes encoding cell surface molecules that interact with immune cells, including cytokines, chemokines, and adhesion molecules.