Review Question - QID 213240

An 11-year-old boy is brought to the emergency department by his parents with a 2-day history of fever, malaise, and productive cough. On presentation, he is found to be very weak and is having difficulty breathing. His past medical history is significant for multiple prior infections requiring hospitalization including otitis media, upper respiratory infections, pneumonia, and sinusitis. His family history is also significant for a maternal uncle who died of an infection as a child. Lab findings include decreased levels of IgG, IgM, IgA, and plasma cells with normal levels of CD4 positive cells. The protein that is most likely defective in this patient has which of the following functions?