T Cells Are a Heterogeneous Population of Cytotoxic T Cells and T-Helper Cells
All T cells express a T-cell antigen receptor (TCR), CD28 and related molecules, and either CD4 (helper cells) or CD8 (cytotoxic cells). TCR specifically binds to antigenic peptides that are presented by APCs.
Based on discrete functions of T cells, these cells are subdivided into two major types: (1) Iielper cells and (2) cytotoxic cells. T-helper (Th) cells secrete proteins called cytokines that act on other immune cells to provide help and coordinate immune responses. Th cells express the CD4 receptor. Cytotoxic T cells kill infected or abnormal cells. These cells express the CD8 molecule (but not CD4) and have granules that are rich in serine esterase granzymes. Cytotoxic T cells also have perforins and Iymphotoxins that are important in initiating cytotoxicity.T-helper cells, based on the predominant cytokines secreted, are further divided into three major types: Th-1, Th-2, and Th-17. Th-I cells predominantly secrete interleukin-2 (IL-2), interferon-gamma (IFN-γ), and tumor necrosis factor beta (TNF-β). Th-I immunity is critical for defense against intracellular pathogens (viral, bacterial, or protozoal) and certain types of tumors. Failure to generate Th-I cells creates susceptibility to these infections. Abnormal activation of Th-I cells can result in a wide variety of inflammatory conditions, including autoimmune states. Th-2 cells predominantly secrete IL-4. IL-6, IL-5, and IL-10. Formation of Th-2 cells is essential for defense against extracellular pathogens, neutralization of toxins and viruses in body fluids, and activation of other cells of the immune system. Abnormal regulation of Th-2 cells leads to allergies. Th-17 subsets predominantly secrete interleukin-17 (IL-17), a cytokine now recognized as an important mediator of inflammatory and autoimmune diseases.
ITius it is clear that the immune system must initiate the correct type of immune response to maintain homeostasis and defend the host appropriately against the invasion of different types of pathogens.