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.