A 9-month-old boy is brought to the pediatrician because of fever and cough for 3 days. This is his third episode of pneumonia and otitis media in the past 6 months. His first episode occurred at 7 months of age when he was hospitalized for pneumonia caused by Streptococcus pneumoniae. He was hospitalized again 2 months later for otitis media and pneumonia requiring intravenous antibiotics; culture grew Haemophilus influenzae. He was born at term following an uncomplicated pregnancy and delivery. He completed his primary vaccination series including pneumococcal conjugate and Haemophilus influenzae type b vaccines. He has no history of oral thrush, chronic diarrhea, or unusual viral infections. Family history is noncontributory. His temperature is 38.9°C (102°F). Physical examination shows bilateral tympanic membrane erythema and decreased breath sounds in the right lower lung field. Serum immunoglobulin studies show IgG 145 mg/dL (normal: 200-1000 mg/dL), IgA 12 mg/dL (normal: 15-90 mg/dL), and IgM 18 mg/dL (normal: 25-100 mg/dL).
Which of the following components of the immune system is most likely deficient in this patient?