This timing of symptom improvement with T-lymphocyte reconstitution rather than significant production of IgM/IgA lends further credence to an undefined deficit in cellular immunity pre-HSCT predisposing him to chronic norovirus infection, rather than agammaglobulinemia (noting also that prior IgRT likely contained anti-norovirus antibodies and did not clear infection). This evidence concerns the gene CD40LG and infection.