Different helper T cell subsets secrete different patterns of cytokines when stimulated by antigen. The TH1 and TH2 subsets differ in the secretion of at least eight cytokines, and three or more other cytokine secretion patterns also exist among both mouse and human T cell clones. Several properties of strong immune responses suggest that at least the TH1 and TH2 phenotypes can be present in vivo. As cytokines are major determinants of the functions of the T cells that produce them, these patterns lead to different properties of the T cell subsets. TH1 cells mediate several functions connected with cytotoxicity and local inflammatory reactions, and so these T cells are particularly effective at combating viruses and intracellular bacteria and parasites. TH2 cells are much more effective at stimulating B cells to produce antibody, and so should be more effective against free-living bacteria, and in inducing protective humoral immunity. Antibody and delayed inflammatory reactions are often mutually exclusive during immune responses, and this can be at least partially explained by cross-inhibition of TH1 and TH2 cells. A newly discovered cytokine, IL10, has been implicated as one of the cross-regulatory cytokines, as this TH2 product inhibits cytokine synthesis by TH1 cells.