T cells have important roles in immune responses and function by directly secreting soluble mediators or through cell contact-dependent mechanisms. T cells can be separated into three major groups based on function: cytotoxic T cells, helper T cells (Th), and regulatory T cells (Tregs). Many T cell subsets have been characterized. Differential expression of antigens on the cell surface, as well as their distinct cytokine secretion profiles, provide valuable clues to the diverse nature and function of T cells. Further knowledge of how these T cell subsets are regulated and cooperate with each other will provide us with better tools to treat immune-related diseases.
For example, CD8+ cytotoxic T cells destroy infected target cells through the release of perforin, granzymes, and granulysin, whereas CD4+ T helper cells (ie, Th1, Th2, Th9, Th17, and Tfh cells) have little cytotoxic activity and secrete cytokines that act on other leucocytes such as B cells, macrophages, eosinophils, or neutrophils to clear pathogens. Tregs suppress T-cell function by several mechanisms including binding to effector T-cell subsets and preventing secretion of their cytokines.