<prompt> In Bare Lymphocyte Syndrome, failure of antigen presentation is due to defective expression ofMHC class I genesImmunoglobulinsT and B cell surface receptorsCluster determinants  <response>: Ans. a MHC class I genes Ref. Nelson Textbook of Pediatrics 17th 699; Robbins pathology 7thpg. 244BARE LYMPHOCYTE SYNDROME It is a condition caused by deficiencies in major histocompatibility complex:- Type 1: MHC class I HLA-A, -B, and -C antigens- Type 2: MHC class II HLA-DR, -DQ, and -DP antigens Isolated deficiency of MHC class I antigens, the bare lymphocyte syndrome, is rare.- The resulting immunodeficiency is much milder than in SCID, contributing to a later age of presentation. - Sera from affected children contain normal quantities of MHC class I antigens and ss2 -microglobulin.- But MHC class I antigens are not detected on any cells in the body.- There is a deficiency of CD8 but not CD4 T cells. The bare lymphocyte syndrome, type II MHC class II is not expressed.- The result is that the immune system is severely compromised.- Clinically, this is known as a severe combined immunodeficiency SCID.- MHC class II-deficient patients have a very low number of CD4 T cells but normal or elevated numbers of CD8 T cells. Lymphopenia is only moderate. The MHC class II antigens HLA-DP, DQ, and DR are undetectable on blood B cells and monocytes, even though B cells are present in normal number. Rx: Bone marrow transplant is currently the only treatment.IMMUNODEFICIENCY DISEASESPhagocytic DeficienciesHumoral DeficienciesCell-mediated Combined immunodeficiences Reduced neutrophils Defective phagocytes Adherence defects Chemotactic defects Killing defects X-linked agammaglobu- linemia X-linked hyper-IgM syndrome Common variable hypo- gammaglobulinemia Selective immunoglobulin deficiencies DiGeorge syndrome Reticular dysgenesis Bare lymphocyte syndrome SCID Wiskott-Aldrich syndrome Complement deficiencies 