CD4 and infection: Antibodies (IgA and IgG) (4–6), effector immune cells (CD4+ T cells, macrophages, and neutrophils) (7–9) and cytokines (including IL-17A and IFN-ɣ) (7, 8, 10) have been shown to play critical roles in regulating immune responses to S. pyogenes at the site of infection.