T cells are lymphocytes that, along with B cells, form the adaptive, cell-mediated immune response. There are several different types of T cells:
T cells can also be known as effector T cells, which can be applied to any subtype of T cell and simply means they are ready to respond and are responding to a stimulus. As illustrated above the different T cell subtypes have distinct functions in the immune response against infection.
The T cell receptor
The defining feature of T cells is the expression of the T cell receptor (TCR) on the surface of the cell. TCRs are highly variable so that they are able to recognize different antigens. TCRs are unable to bind antigen directly, the antigen is broken down into peptides and presented to the T cell by antigen presenting cells (APC) such as dendritic cells and macrophages. It is the major histocompatibility complexes (MHC) on the APC surface that hold the antigen/peptide and present it to T cells. MHC class I present to cytotoxic T cells and MHC class II present to helper T cells. For T cell activation to take place, in addition to the TCR binding to antigen peptide presented by MHC, simultaneous binding to a co-receptor molecule is required. CD4 is the co-receptor expressed by helper T cells and CD8 is expressed by cytotoxic T cells, these co-receptors help stabilize the TCR-MHC complex. Further to the MHC-TCR and co-receptor binding co-stimulatory molecules are required for effective T cell activation which then results in the instigation of various signaling pathways. For example co-stimulatory receptor CD28 on the T cell binds to proteins such as B7-2 or B7-1 on APCs. Read our TCR mini-review for detailed information on TCR activation, signaling, development and diversity.
The TCR is composed of chains that are required in the transmission of the signal to the T cell. These chains can either be alpha and beta or gamma and delta. The majority of T cells are composed of alpha and beta chains, like those found on CD4+ helper T cells and CD8+ cytotoxic T cells. Gamma/delta TCR T cells form a small separate subset of T cells with their own set of specific features and functions. For efficient signal transmission through the TCR a further co-receptor is required. This receptor is composed of various epsilon, zeta, delta and gamma chains. When complexed around the TCR this co-receptor is known as CD3. Read our CD3 mini-review for further detailed information about CD3 including its structure alongside that of the TCR, the genes involved in its expression, its function and the signal transduction pathways mediated by CD3 complex, or click here to view antibodies against this important T cell marker.
T cell lineage
T cells are predominantly produced in the thymus; some T cells however mature in the tonsils, hence the name T cells. T cells originate from hematopoietic stem cells, which after several stages of development result in a common lymphoid precursor. It is at this stage of development where B, T and other lymphoid derived cells branch out into separate lineages. T cell development continues through the development of double negative and then double positive thymocytes following on from which the different T cell subtypes arise. Figure 1 below shows the T cell lineage of human helper T cells and cytotoxic T cells alongside key markers for the different stages of development. For further information on T cell development in humans and also in mouse refer to our collection of posters and guides.
Fig 1: Human helper CD4+ T cells (a) and cytotoxic CD8+ T cells (b). Click on the T cell lineage to view human lineage posters and guides. Mouse lineage posters and guides are also available.
Bio-Rad provides a comprehensive range of antibodies for T cell research, either search for these using the filter table below or by entering your marker in the search box at the top of the page.