Most patients with X-linked hyper-IgM present in the first 2 years of life with recurrent sinopulmonary infections, while some patients have presented with Pneumocystis carinii pneumonia. Low or undetectable serum IgG, IgA and IgE levels with a normal or elevated IgM level in a boy suggests the diagnosis of X-linked hyper-IgM. The presence of a normal number of mature B cells in X-linked hyper-IgM differentiates it from X-linked agammaglobulinemia. Normal numbers of T cells and T cell subsets with normal T cell proliferation to mitogens differentiates X-linked hyper-IgM presenting with P. carinii infection from severe combined immunodeficiency or HIV infection. Identification of a defect in the CD40L
gene confirms the diagnosis of X-linked hyper-!gM. Patients with X-linked hyper-IgM have been found to produce specific antibodies in response to vaccination, but the antibodies are restricted to the IgM isotype. Patients lack germinal centers in their lymph nodes and do not exhibit memory responses to repeated immunizations. Because the genetic defect is not lethal to T cells, carriers have random inacti-vation of the X chromosome in their T cells. As few as 30% of the carriers' CD4 T cells may express normal CD40L with no apparent effect on immune function (Figure 1).
Patients with X-linked hyper-IgM are also susceptible to infections with Histoplasma capsulatum and Cryptosporidium; the latter is associated with chronic diarrhea. Other manifestations of the disease include recurrent episodes of stomatitis that may be associated with neutropenia. The cause of the neutropenia is not known; it may be secondary to the development of antineutrophil antibodies (although such antibodies are usually not detected) or to a defect in myeloid differentiation. The presence of autoantibodies has been reported in X-linked hyper-IgM and may lead to the development of anemia, thrombocytopenia or nephritis. Arthritis may result from autoantibodies or be secondary to viral or mycoplasmal infections. Patients have an increased risk of developing B cell lymphoproliferative disease that is usually polyclonal but nevertheless can be fatal.
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