immunology allergic disorders
Severe Combined Immunodeficiency (SCID)
Introduction
Severe combined immunodeficiency is characterized by low to absent T cells and a low, high, or normal number of B cells and natural killer cells. Most infants develop opportunistic infections within the first 3 months of life. Diagnosis is by detecting lymphopenia, absence or a very low number of T cells, and impaired lymphocyte proliferative responses to mitogens. Patients must be kept in a protected environment; definitive treatment is hematopoietic stem cell transplantation. (See also Overview of Immunodeficiency Disorders and Severe combined immunodeficiency (SCID) is a Primary Immunodeficiencies that involves There are 4 different abnormal lymphocyte phenotypes: B cell positive, NK cell negative B cell negative, NK cell positive B cell negative, NK cell negative B cell positive, NK cell positive In most forms of SCID, T cells are absent (T-); the number of B cells and/or natural killer (NK) cells may be low or none (B-; NK-) or high or normal (B+; NK+), depending on the form of SCID. However, B cells, even when normal in number, cannot function because T cells are absent. Natural killer cell function is usually impaired. The most common form of SCID is X-linked. It affects the interleukin (IL)-2 receptor common gamma chain (a component of at least 6 cytokine receptors) and thus causes severe disease; phenotype is T- B+NK-. It results from a mutation in the IL-2 receptor gamma gene (IL-2RG). The 2nd most common form results fromadenosinedeaminase (ADA) deficiency, which leads to apoptosis of precursors for B, T, and NK cells; phenotype is T- B- NK-. The next most common form results from IL-7 receptor alpha-chain deficiency; phenotype is T- B+NK+. Mutations in recombination activating gene 1-2 (RAG1orRAG2) represent a T-B-NK+ SCID phenotype. Omenn syndrome is another T-B-NK+ SCID phenotype. It typically is the result of one defective RAG allele and therefore is commonly referred to as atypical SCID, or leaky SCID. It is an autosomal recessive form of SCID. Frequently the levels of IgA, IgG, and IgM are low while there is elevated IgE and eosinophilia. These patients usually have a varying degree of lymphopenia with low numbers of T and B cells and present with inflammation and lymphadenopathy. Artemis-deficient SCID is a rare type that occurs primarily in children of Apache or Navajo descent. It results from a mutation inDCLRE1C. 