Basophils amplify IL-17 production by effector (T_EM) and central memory (T_CM) CD4 T cells. T_EM, T_CM, or CCR6⁺ CD4 T cells were cultured with or without autologous or allogeneic basophils (at a 5:1 ratio) in the presence of (A-D) IL-3 or IL-33 (E-F). (A) IL-2–stimulated T_EM. (B) T_EM stimulated by anti-CD3/28–coated beads. (C) T_CM stimulated by anti-CD3/28–coated beads. (D) CCR6⁺ CD4 T cells stimulated by IL-2. (E-F) T_EM stimulated by IL-2 and T_CM stimulated by anti-CD3/28–coated beads. (A-F) After 4-5 days, the proportion of IL-17–producing CD4 T cells was measured by intracytoplasmic staining (1 representative dot plot) and IL-17 and IFN-γ secretion were measured by ELISA. *P < .05; **P < .01.