What is the major role of HLA class III region genes?

HLA class III contains genes for early complement components C2 and C4 of classical pathway. Deficiency of these early components of the classical pathway is associated with autoimmune disease like SLE and other collagen vascular diseases. These genes are thus impoant in regulating susceptibility to autoimmune disease. Class III genes do not paicipate in MHC restriction or graft rejection components. Many proteins coded by MHC class III genes may be involved in the development of autoimmunity.Ref: Textbook Of Immunology By Basir, Page 93-4; Essential Immunology By Roilt, Page 262; Textbook of Microbiology By Ananthanarayan and Panicker, 6th Edition, Page 108.