Lymphocytes (i.e WBC's) can be divided into T cells, B cells and Natural Killer cells. The role of T cells is to activate the immune system, directly kill the antigen containing cell and produce cytokines to alert other immune cells in the body. T cells can further be divided into three subsets based on their functions. The three subsets are Cytotoxic T cells (CD8+), Helper T cells (CD4+) and Regulatory T cells. The function of Cytotoxic T cells is to kill the target cell by injecting cytotoxic granules into the infected cell. Helper T cells activate other phagocytic cells by releasing cytokines. Regulatory T cells down regulate the immune system.