Many immunodeficiencies that selectively involve one or a few Ig isotypes have been described. The most common is selective IgA deficiency, which affects about 1 in 700 Caucasians and is thus the most common primary immunodeficiency known. IgA deficiency usually occurs sporadically, but many familial cases with either autosomal dominant or recessive patterns of inheritance are also known. The clinical features are variable. Many patients are entirely normal; others have occasional respiratory infections and diarrhea; and rarely, patients have severe, recurrent infections leading to permanent intestinal and airway damage, with associated autoimmune disorders. IgA deficiency is characterized by low serum IgA, usually less than 50 |g/mL (normal, 2 to 4 mg/mL), with normal or elevated levels of IgM and IgG. The defect in these patients is a block in the differentiation of B cells to IgA antibody-secreting plasma cells. The a heavy chain genes and the expression of membrane-associated IgA are normal. No gross abnormalities in the numbers, pheno-types, or functional responses of T cells have been noted in these patients. In a small proportion of patients with selective IgA deficiency, mutations have been described in TACI (transmembrane activator and calcium modulator and cyclophilin ligand interactor), one of the three types of receptors for the cytokines BAFF (B cell-activating factor) and APRIL (a proliferation-inducing ligand). TACI mutations are also an important cause of common variable immunodeficiency, discussed later. IgA deficiency may represent a forme fruste of common variable immunodeficiency.
Selective IgG subclass deficiencies have been described in which total serum IgG levels are normal but concentrations of one or more subclasses are below normal. Deficiency of IgG3 is the most common subclass deficiency in adults, and IgG2 deficiency associated with IgA deficiency is the most common in children. Some individuals with these deficiencies have recurrent bacterial infections, but many do not have any clinical problems. Selective IgG subclass deficiencies are usually due to abnormal B cell differentiation and rarely to homozygous deletions of various constant region (CY) genes.
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