This pattern is consistent with the canonical role of IL-4 (and IL-13) as the key “first signal” driving class-switch recombination to IgE in human B cells (41) and with data showing that IL-9 further amplifies IL-4-dependent IgE and IgG production by B cells and promotes mast cell expansion in IgE-driven allergic disease (42). Here, IL9 is linked to allergic disease.