T cells

T-cells or T-lymphocytes are a group of lymphocytes involved in specific immune response, specifically antigen-specific cellular interactions. There are two main subsets; Cytotoxic T-cells and Helper T-cells, Helper T-cells are further divided into inflammatory (TH1) and (TH2) Helper cells which assist B-cells by forming antibodies. Cytotoxic T-cells are also known as CD8+ T cells, and Helper T-cells are also known as CD4+ T cells. T-cells are formed from bone marrow stem cells that undergo maturation in the Thymus.

Individual T cells are able to recognise only certain antigens, discriminating between antigens using protein molecules on the cell surface called receptors. The receptor and the antigen fit together like a lock and a key only when their shapes match perfectly. The number and specificity of T cell receptors appear to be determined by the cell’s genes.

Before T cells have encountered an antigen, they are known as naive T cells and they have a low affinity for Interleukin-2 (IL-2). Naive T cells enter the lymph node from blood via high endothelial venules (HEV), to move to a T cell area rich in dendritic cells and macrophages (APC). APC present specific antigens to the T cell, and deliver other activation signals (e.g cytokines). Once they recognise an antigen they become activated and once activated they have a high affinity for IL-2 which leads to lots of IL-2 binding which causes a proliferation of T cells (binding of IL-2 causes T cells to begin the cell cycle). T cells which are not activated (i.e. they did not encounter any antigen or MHC), leave the lymph node via cortical sinuses into the lymphatics to re-enter cirulation.

Activation of T cells by antigen recognition also causes them to differentiate into 3 different types of T cells. CD8 T cells differentiate into Cytotoxic T cells and CD4 T Cells differentiate into CD4 TH1 and CD4 TH2 cells.

CD4+ T cells recognise peptides bound to MHC class II complexes bound on the cell surface, whereas CD8+ T cells recognise peptides bound to MHC class I complexes bound to the cell surface.