IL17A and infection: Traditional helper cell functions during infection include TNF-α and IFN-γ secretion that activates other immune cells and can protect against intracellular infections (Th1), IL-4-induced class switching to IgE, and protection against extracellular worm infections (Th2), IL-17, and IL-22 secretion that promotes protection against extracellular bacterial and fungal infections (Th17), and IL-21 secretion that facilitates germinal center formation and class switching to protein antigens (Tfh) (1, 2).