IL4 and immunoglobulin G4-related sclerosing disease: T follicular helper (Tfh) cells have a particularly important role in allergy by promoting isotype switching to IgE via secretion of IL-4 (29) and in IgG4-RD by contributing to isotype class switching and promoting ectopic GC formation through IL-21 production (Figure 1 and Table 1) (23, 30).