TH cells secrete cytokines for only a short period after they have been activated; the range of cytokines that TH cells secrete after activation chiefly determines their function. Different T-helper cell subpopulations (Th<sub>1</sub> and TH<sub>2</sub> cells) secrete different sets of cytokines. | TH cells secrete cytokines for only a short period after they have been activated; the range of cytokines that TH cells secrete after activation chiefly determines their function. Different T-helper cell subpopulations (Th<sub>1</sub> and TH<sub>2</sub> cells) secrete different sets of cytokines. |