T cells are lymphocytes that mediate the adaptive immune response through three primary subsets: Cytotoxic, Helper, and Regulatory T cells. Cytotoxic T cells (CD8+) directly destroy infected or abnormal cells by releasing cytotoxic granules. Helper T cells (CD4+) secrete cytokines that activate and coordinate other immune cells, such as macrophages and B cells. Regulatory T cells downregulate the immune response to maintain homeostasis and prevent autoimmunity.

State University of New York at Buffalo

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T lymphocytes, which are commonly called T cells, express T cell receptors (TCRs) on their surface.  These are transmembrane proteins that bind to processed antigens that are presented by antigen presenting cells (ATCs).  
There are three types of T cells, differentiated on the basis of their functions:
1. Cytotoxic T- cells
2. Helper T cells 
3. Regulatory T cells

T Lymphocytes

Cytotoxic T cells are CD8+ T lymphocytes whose primary function is to directly destroy infected or abnormal target cells. They recognize specific antigens presented by MHC class I molecules on target cells. Upon recognition, cytotoxic T cells release cytotoxic substances (such as granzymes) from their granules to destroy the target cell.

Function of Cytotoxic T cells

Helper T cells are known to be CD4+. Their basic function is to produce cytokines that up regulate the immune system and alert the other phagocytic cells. Helper T cells can be further divided into TH1, TH2 and TH17 cells. TH1 cells stimulate the CD8+cells and macrophages to kill the infected cells.TH2 cells stimulate B cells to produce antibodies. TH17 cells help maintain the mucosal barrier and is involved in auto-immunity. They are also involved in pro-inflammatory response as they recruit neutrophils and macrophages to site of infection. T helper cells recognise MHC class II molecules.

Function of Helper T cells

Regulatory T cells are CD4+,CD8+ and FoxP3+. The basic function of these cells is to down regulate the immune system. These cells are important for the "self check mechanism" to prevent auto immune diseases. 

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