Cell Mediated Immunity

T cell function in adaptive immunity involves effector TH cells which provide cytokine help to B cells for antibody production against T-dependent antigens. A second T cell function involves cytotoxic T cells; CTLs in T cell immunity. CTLs mediate killing of altered host target cells infected with intracellular pathogens or neoplastic cells. T cell immunity mediated by CTLs involves the killing of altered host cells by apoptosis. T cell immunity is also referred to as cell-mediated immunity, and involves the activity of cytotoxic effector cells of different types. Cytotoxic T cells (Tc) also called precursor cytotoxic T lymphocytes (CTL-P) are the predominant cell in T cell immunity, aided by T helper cells, in particular TH1 cells. Cytotoxic T cells are CD8+ and have two known subsets, Tc1 and Tc2 differentiated by the cytokines they secrete and the mode of killing employed when interacting with altered host cells. Similar to TH1 and TH2 cells, Tc1 cells secrete IFNγ and Tc2 cells secrete IL-4 and IL-5. Tc1 cells kill by using both the Fas-FasL and perforin-granzyme pathways while Tc2 cell use only the perforin-granzyme pathway. The targets of CTL effects are intracellularly infected host cells, tumor cells, and cells bearing non-self MHC class I proteins (allogeneic antigens) on transplanted tissue. Other cells involved in cytotoxicity include, NK cells, NKT cells and macrophages.