Adaptive immunity has both a humoral and a cellular arm. T-cells are usually associated with the cellular arm, and B-cells with the humoral arm. In fact, T-cells have essential roles in humoral, as well as in cellular immunity. Helper T-cells (Th) perform the following functions: They activate both B cells and T cells They release cytokines that promote immune cell proliferation and attract leukocytes into areas of inflammation. They also play a role in innate immunity by activating macrophages. Cytotoxic T cells (Tc cells) directly attack and kill infected cells, cancer cells, and foreign cells , for example, cells from blood transfusions or tissue grafts.
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Like B cells, T cells are produced by the erythropoietic tissue of the bone marrow. But unlike B cells, T cells go to the thymus to mature and become activated. In addition to their antigen receptors, T cells have CD4 and CD8 glycoprotein receptors on their cell membranes which play a role in the interaction between T cells and other cells. CD4 cells differentiate ( after exposure to antigens on APC cells) into T helper cells (Th) and T regulatory cells (Tr). T helper cells help to activate B cells; they also help to activate other T cells. Generally, they may be said to direct the adaptive response. In addition to these activities CD4 cells also differentiate into T regulatory (T r ) cells that moderate immune responses and keep them in balance. T cells with CD8 receptors differentiate into T cytotoxic (Tc ) cells. T cytotoxic cells are killers that destroy any cell that contains foreign substances or organisms. Memory T-cells. Just as there are memory B cells, there are memory T cells that differentiate from CD4 and CD8 cells . These memory Th, Tc and Trg cells are reserved for quick responses to challenges. subsequent to their formation. Three important subsets of T helper cells are T1, T2 and T17 T helper cells . They have the following specific activities: TH1 cells mediate cellular immunity--stimulate the differentiation of other T cells , and activate macrophages. TH2 cells ; Stimulate humoral responses that depend on antibody formation; defend against parasitic; promote allergic responses. TH 17. Th 17 cells link innate and adaptive responses; release interleukin 17; promote immune responses against extracellular microbes, and mediate autoimmunity.