Agammaglobulinemias

There are three major categories of antibody deficiencies: (a) defects in early B cell development, (b) hyper-IgM syndromes (also called class switch recombination defects), and (c) common variable immunodeficiency (CVID). Category (a) consists of agammaglobulinaemias. Defects in early B cell development are characterized by the onset of recurrent bacterial infections in the first 5 years of life, profound hypogammaglobulinemia, markedly reduced or absent B cells in the peripheral circulation, and (in the bone marrow) a severe block in B cell differentiation before the production of surface immunoglobulin-positive B cells. Mutations in Btk, the gene responsible for X-linked agammaglobulinemia (XLA), account for approximately 85% of affected patients. Approximately half of the remaining patients have mutations in genes encoding components of the pre-B cell receptor (pre-BCR) or BCR, including mu heavy chain (IGHM); the signal transduction molecules Ig-alpha (CD79A) and Ig-beta (CD79B); and lambda 5 (IGLL1), which forms the surrogate light chain with Vpre-B. A small number of patients with defects in BLNK, a scaffold protein that assembles signal transduction molecules activated by cross-linking of the BCR, have been reported.