7. A 2 yr old boy presents with recurrent bacterial infections and Pneumocystis jiroveci pneumonia. Labs show markedly elevated IgM and low IgG. Which of the following is the most likely cause of his illness?

The history and labs given here are suggestive of hyper IgM syndrome - An inherited deficiency in the CD40 ligand or CD 40. The X-linked form affects helper T cells which do not express functional CD40L on their plasma membrane. The autosomal recessive form affects CD40 on B cells. Costimulatory engagement of CD40 with CD40 ligand is required for activation of B-cells. In hyper-IgM syndrome, class switching of B-cells is affected, leading to high IgM and low levels of IgG. B-cell responses to Tdependent antigens and production of memory cells is also deficient. The B-cell response to T-independent antigens is unaffected. Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition CD40-CD40L interactions are also required for IL-12 secretion; so, defects in this pathway result in increased susceptibility to intracellular pathogens.