X-linked hyper-IgM syndrome is characterized by a deficiency of IgG, IgA, and IgE and elevated levels of IgM. IgM levels can reach 2,000 mg/dL (normal 45–250 mg/dL). It is most commonly inherited as an X-linked recessive disorder, but some forms seem to be acquired and can be seen in both sexes.

• Peripheral blood of patients has high numbers of IgM-secreting plasma cells, as well as autoantibodies to neutrophils, platelets, and red blood cells.

• Patients fail to make germinal centers during a humoral immune response.

• Children with this condition suffer recurrent respiratory infections,

  especially those caused by Pneumocystis jirovecii.

  The defect in this syndrome is in the gene encoding the CD40 ligand, which maps to the X chromosome. Therefore, Th cells in these patients will fail to express functional CD40L on their membrane, failing to give the costimulatory signal needed for the B-cell response to T-dependent antigens. As a result, only IgM antibodies are produced

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