A 9-month-old boy presents to clinic with his third episode of otitis media in the last 2 months. His other medical history includes pneumonia at the age of 7 months, for which he was hospitalized. Blood cultures at that time grew Haemophilus influenzae. His growth and physical development have been normal. Family history reveals a male cousin who died at age 14 months from pneumonia. On physical examination, there is no lymphadenopathy, and tonsillar tissue is not visualized. He has a bulging, erythematous tympanic membrane on the left. Lungs and remainder of physical examination are normal. You suspect immune deficiency and perform extensive laboratory workup. Flow cytometry reveals few circulating B-lymphocytes.