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A small 9-month-old male with a history of recurrent pyogenic infections is seen in a clinic. Immunoglobulin levels and a CBC are performed. The CBC is normal except for slight neutropenia and thrombocytopenia. Determination of immunoglobulin levels indicates elevated IgM, but deficiencies of IgG and IgA. The underlying defect involves which of the following molecules?
A. CD40 ligand (CD40L) on the T cell
B. CD40 molecule on the B cell
C. Gamma interferon
D. Interleukin-2 (IL-2)
E. Interleukin-3 (IL-3)
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CORRECT
The correct answer is A. Patient's with hyper-IgM syndrome (HIGM) experience very little, if any, isotype switching. The B cells in these patients cannot undergo the switch from IgM to IgG, IgA, or IgE that normally occurs during B-cell maturation. When B cells undergo isotype switching they require two factors: IL-4, which binds to a specific receptor on the B cell, and the CD40 molecule on the B-cell surface, which binds to the CD40 ligand (CD40L) on an activated T-cell surface. The deficiency is due to mutations in the CD40L. This immunodeficiency results in patients who are IgG- and IgA-deficient, but synthesize large amounts of polyclonal IgM. Affected individuals are susceptible to pyogenic infections, and often form IgM autoantibodies to neutrophils, platelets, or tissue antigens. The disease is inherited as an X-linked recessive in 70% of the cases.
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specially susceptible against pneumocystis carinii
