a)     Richard is a 12-month-old infant with a severe gram-negative kidney infection (pyelonephritis). This is his third such infection in 6 months, although investigation of the urogenital system for congenital defects (e.g., ureteral valvular defects) revealed no abnormalities. The mother insists ‘something’ must be wrong because he has had recurrent sinus and pulmonary infections and repeated episodes of diarrhea. Richard has three healthy sisters aged 1, 3, and 4 years. An uncle died at 10 years of age with bacterial pneumonia. Despite the fact that Richard had received childhood immunizations, blood tests performed at a hospital revealed low serum IgG, IgA, and IgE levels, but high serum levels of IgM. B and T cell numbers were normal, as was the stimulated function of T cells. There was no evidence for defective neutrophil functioning.