Cytotoxic T cells are known to be CD8+. CD markers are surface proteins used to identify immune cells. The function of cytotoxic T cells is to directly kill the antigen infected cell by releasing certain cytotoxic substances in the cell which include granzyme and lysozyme. Cytotoxic T cells recognise MHC class 1 molecules that act as antigen presenting cells for the CD8+ cells. 

State University of New York at Buffalo

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

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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