<prompt> HLA B 27 is not seen inSLEAnkylosing spondylitisJuvenile rheumatoid ahritisReactive ahritis  <response>: Major histocompatibility complex:Required for recognition of antigens by T cellsEncoded by genes located on chromosome 6. The genes encoding MHC is categorized into 3 classes.Class I contains three loci namely A, B and C. Class II contains 3 loci namely DP, DQ and DR. Class III contains genes that produce complement proteins and TNF. Class I MHC: Expressed on all nucleated cells and platelets. The receptor comprised of 4 molecules - 3 alpha and 1 beta2 microglobulin. Peptide binding cleft is formed by alpha1 and alpha 2 which display viral exogenous antigens. Present in CD 4 positive T lymphocytes.Class II MHC: Expressed on antigen presenting cells macrophages, B lymphocytes, and dendritic cells and on CD8 positive T lymphocytes. The receptor comprised of two alpha and 2 beta domains. Peptide binding cleft is formed by alpha 1 and beta 1 domains which display endogenous antigens. Present in CD8 positive T cells.HLA -B27 is a class I MHC molecule which is present in all nucleated cells, hence B cells and T cells are excluded.Inflammatory diseases associated with HLA B27 are ankylosing spondylitis, Reactive ahritis, Juvenile rheumatoid ahritis and acute anterior uveitis. Ref: Robbins 8e p190-193 