T-cell

Overview
  • T-cells are a subtype of lymphocyte which are the main effector of Cell-mediated Immunity but also participate in the coordination of Humoral Immunity. These leukocytes are distinguished from other cells by their membrane expression of T-cell Receptor. These cells originate in the bone marrow but undergo their major development within the thymus.
Subtypes
  • Two major classes of T-cells have been recognized for many years based on their expression of certain T-cell Coreceptor membrane molecules, CD4 and CD8. T-cells which express CD4 are termed CD4+ T-cells and those which express CD8 are termed CD8+ T-cells. CD4+ T-cells, which are also referred to as "Helper T-cells" or "Th Cells" possess their own functional sub-classification into Th1 and Th2 Cells. These different subtypes of T-cells participate in different arms of the immune response as discussed in the section below
  • FYI: It should be pointed out that additional physiologically important classes of T-cells have been discovered in recent years such as Th17 cells which express IL-17 and regulatory T cells (Treg) which express CD25 and FOXP3; however, these are relatively new findings in immunology and will not be discussed further in this text
Function
  • Overview
    • The role of the following T-cell subsets are discussed in much greater detail in Cell-mediated Immunity and Humoral Immunity. Below we only provide brief description of basic functionality.
  • CD8+ T-cells
    • CD8+ T-cells are the main effector cells of Cell-mediated Immunity. These cells recognize and kill microbe-infected cells and are thus critical for immunity against intracellular pathogens.
  • CD4+ T-cells
    • Th1 Cells
      • Th1 Cells can provide additional stimuli to macrophages which have phagocytosed microbes to kill the internalized bacteria. This largely occurs by Th1-mediated secretion of Interferon-gamma onto the macrophage.
    • Th2 Cells
      • Th2 Cells are critical for coordination of Humoral Immunity. These cells induce antigen-specific B-cells which have encountered microbial antigen to proliferate as well as switch their antibody isotype. These functionalities are in part carried out through Th2-mediated secretion of IL-4 and IL-5 onto activated B-cells.
Development
Morphology
  • T-cells, like all lymphocytes possess a large, unlobulated nucleus, justifying their classification as Mononuclear Cells. The nuclei of lymphocytes generally takes up the majority of the cell and thus they possess little in the way of cytoplasm. Inactive T-cells are generally small, about the size of an erythrocyte, whereas active T-cells are nearly twice as large.