An 25-year-old man comes to the physician for a 2-day history of fever and right-sided chest pain. Physical examination shows a right lower lobe consolidation with dullness to percussion. Temperature is 101 F. He has had two such illnesses per year for the past decade. Two months ago, he had a sore cheek and fever that resolved with antibiotics. Deficiency of which of the following is the most likely explanation for this patient’s presentation?
CD40 ligand
Cell-mediated immunity
Mucosal immunoglobulin
Interleukin-2 receptor
Tyrosine kinase
