Review Question - QID 215191

A 2-year-old boy is brought to a pediatrician to establish care after his family recently moved to the United States. His parents say that he was born healthy and at term with no major neonatal complications. He regained his birth weight at 2 weeks, had separation of the umbilical cord at 2 months, and started walking at 10 months. His parent's main concern is that he gets sick significantly more often than his siblings did at the same age. Specifically, he gets recurrent middle ear infections as well as hard-to-eradicate skin infections. Physical exam reveals red, inflamed gums with numerous ulcerations of the oral mucosa. Which of the following immune system processes is most likely defective in this patient?