The document discusses the major histocompatibility complex (MHC), which controls a major part of the immune system. It defines MHC as a set of cell surface proteins expressed on all nucleated cells and encoded by a large gene family. MHC molecules play a role in antigen presentation, autoimmune diseases, and transplantation. MHC genes in humans are found on chromosome 6 and are divided into three classes - class I expressed on all tissues, class II expressed mainly by antigen presenting cells, and class III encoding complement and TNF proteins. MHC molecules participate in discriminating self from non-self and in both humoral and cell-mediated immunity by presenting antigens.