B cell deficiency diseases

The physiological importance of B cell function is revealed by diseases that result from selective B cell deficiencies and consequent lack of antibodies (agammaglobulinemia). Bruton's agammaglobulinemia (XLA) is an X-linked defect in B cell maturation in humans, with arrest at the Pre-B I stage and a resulting deficiency in all immunoglobulin classes. A corresponding B cell maturation defect, the Xid mutation, also occurs in CBA/N mice. In male children with XLA, the maturation block results from deficiency of a B cell-specific protein tyrosine kinase, btk. With a profound lack of mature B cells but normal T cells, these children are particularly susceptible to infections by bacteria, mycoplasma, hepatitis virus and enteroviruses. They have recurrent middle ear infection, pneumonia, sinusitis and tonsillitis caused by Pneumococcus, Streptococcous and Hemophilus. Problems with infection begin several months after birth, when the pool of protective maternal antibody decreases. XLA children are not unusually susceptible to most virus or fungal infections, for which T

cell responses provide sufficient protection. In contrast, subjects with profound defects of both B cells and T cells (severe combined immunodeficiency, SCID), are threatened by all infectious agents; they survive only in a germ-free environment unless the deficiency can be treated.

Immunodeficiency with elevated IgM but very low IgG or IgA levels results from a failure in the signaling that instructs B cells to switch from production of IgM to other classes of Ig. The signals come in part from T cells, and in most subjects the basic defect is in the T cell even though it is expressed through B cell dysfunction. An X-linked type of this disease is due to a mutation in the gene for CD40L, the T cell ligand for B cell CD40. High levels of IgM in this disease are associated with autoimmunity. A non-X-linked immunodeficiency, in which there is a selective lack of IgA-producing B cells, leads to bacterial infections primarily in the respiratory, gastrointestinal and urogenital tracts.

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