HLA-C and autoimmune disease: The MHC is divided into 3 functionally distinct regions: class I and II regions contain highly polymorphic human leukocyte antigen (HLA) genes that are strongly associated with risk for autoimmune disease 19, 20, 21, and the class III region contains complement component 4 (C4) genes, which are strongly associated with risk for schizophrenia (22).