Ans. C: Auto-immune diseasesThe major histocompatibility complex (MHC) is a large genomic region or gene family.It is the most gene-dense region of the mammalian genome and plays an impoant role in the immune system and autoimmunity.The proteins encoded by the MHC are expressed on the surface of cells and display both self antigens (peptide fragments from the cell itself) and nonself antigens (e.g., fragments of invading microorganisms) to a type of lymphocytes called a T cell that has the capacity to kill or coordinate the killing of pathogens in infected or malfunctioning cells The classical MHC molecules (also referred to as HLA molecules in humans) have a vital role in the complex immunological dialogue that must occur between T cells and other cells of the body. The polypeptides may be "self," that is, originating from a protein created by the organism itself, or they may be foreign ("nonself"), originating from bacteria, viruses, pollen, and so onThree main sets of genes are suspected in many autoimmune diseases. These genes are related to:Immunoglobul insT-cell receptorsThe major histocompatibility complexes (MHC).Ceain MHC class 11 allotypes are strongly correlated withHLA DR2 is strongly positively correlated with Systemic Lupus Erythematosus, narcolepsy and multiple sclerosis, and negatively correlated with DM Type 1.HLA DR3 is correlated strongly with SjOgren's syndrome, myasthenia gravis, SLE, and DM Type 1.HLA DR4 is correlated with the genesis of rheumatoid ahritis, Type 1 diabetes mellitus, and pemphigus vulgaris.Fewer correlations exist with MHC class I molecules. The most notable and consistent is the association between HLA B27 and ankylosing spondylitis.