IGHE and hyperinsulinemic hypoglycemia, familial, 4: We report that while T-bet is largely redundant for in vivo functions of human B cells and humoral immunity, it has a nuanced role in regulating Ig CSR, evidenced by increased serum levels of IgG1, IgG4 and IgE, reduced serum IgG2 levels, and increased proportions of IgG1+ and IgG4+ memory B cells, in a patient with complete T-bet deficiency.