|Current rating - 0|
|Ratings - 0|
From Sci-Mate WikiBiological and Medical Sciences > Classification by Discipline > Immunology
Type: General Knowledge
Intention: For Publication
Read/Write Permissions: Open Access
16.04.2009 - - U. Edinburgh
09.04.2009 - Christopher Dyer - Sci-Mate
T cells or T lymphocytes are a type of white blood cell, characterised by the presence of the T Cell Receptor (TCR) on their surface. They are particularly important compenents of the acquired immune system, influencing both cellular and humoural responses. Their primary function is to eliminate pathogens by patroling the body searching for antigens presented on cell surface MHC complexes. The study of T cells and how they are regulated has been critical in helping scientists find ways to manage a large number of autoimmune, infectious and oncological diseases.
 Activation and Regulation
Activation of T cells generally requires both an antigen specific interaction with the TCR (signal 1), plus a scond signal (signal 2 or costimulatory signal), such as the one induced by the molecule CD28. Naive T cells (that have never been activated before) have to be activated by specific cells that sense the 'danger signal' called Dendritic cells. Activation generally cause T cells to modify their phenotype through the expression of cytokines and membrane proteins, and to proliferate. The regulation of T cells, therefore, is essential to orchestrate a correct immune response. A lack of regulation can result in T cells binding to self-antigens and killing host cells (ie. autoimmune diseases); while excessive regulation of the immune response leads to uncontrolled disease progression (eg, tumors, and chronic infections).
Various types of T cells have been characterized, mainly by profiling their phenotype based on what proteins are expressed at their cell surface (which generally influences cell function). The three major T cell subtypes are CD4, CD8 and Regulatory T Cells.
 CD4 T Cells / Helper T Cells
Cells carrying the CD4 ligand are a major class of T cells known as either CD4 T Cells, CD4+ T Cells, or T 'helper" cells. As suggested by their name they 'help' the immune system respond correctly in response to self and foreign antigens by regulating the behaviour of other cells.
 CD8 T Cells / Cytotoxic T Cells
Cytotoxic T cells, or CD8+ T Cells (with the CD8 phenotype), recognise antigen in the context of MHC Class I, and if activated will kill the bound cell.
 Regulatory T cells
Regulatory T Cells are a relatively new classification of T cells involved in restraining responses during processes such as tolerance, anergy, and latency.
Several subsets of Treg have also been identified, such as 'natural' Treg expressing high levels of CD25 and Foxp3. These cells will inhibit the activation of other types of T cells by 'touching' them (contact dependent). Inducable Tregs such as Tr1 or Th3 cells regulate the ongoing immune response through the release of cytokines, such as IL-10 and TGFβ respectively.