T cell or T lymphocyte is one of the immune cells that grow in the thymus gland. It performs a significant role in stimulating an immune response. T lymphocytes are different from the other immune cells by having a T-cell receptor on its cell surface. It originates from the stem cells of bone marrow as the precursor cells, and later these move into the thymus gland where they get differentiated into several distinct types of T.
The differentiated T cells leave the thymus and goes into the lymphatic system or peripheral tissues, in search of an antigen on the surface of antigen-presenting cells.
Content: T Cell
- T Cell Receptor
- T Cell Maturation
T cells are the white blood cells that are the essential components of an adaptive immune system, which differentiates into specific helper, regulatory, cytotoxic T cells and later becomes the memory T cells. It provides a variety of immune-related functions, like antigen recognition, regulation of immune response, stimulates the production of antibodies by the B cells, cell apoptosis etc. There are millions of T cells, which possess unique receptors on its surface that acts against the specific antigens.
The types of T cell depends upon the cell receptors, functions and the type of antigens encountered. These are broadly categorized into regulatory and effector T lymphocytes based on the presence of co-receptors like CD8+ or CD4+. T lymphocytes differentiate into cytotoxic T cells that possess CD8 surface marker. T cells differentiate into helper T cells that possess CD4 surface marker.
Cytotoxic T Cells
It also refers as cytolytic or CD8+ T cells that consist of CD8+ as surface markers and recognize the target cell via MHC Class I restriction. Cytotoxic T cells primarily kill their target cells by producing cytokines. The target cells include tumour cells, allograft and virus affected cells.
By the association of dendritic cells and T helper cells, it receives signal to kill the target or infected cell. Therefore, it plays a central role in destroying the cells not only infected with the virus but also kill the cells infected with intracellular bacteria or tumorous cells.
Helper T Cells
It also refers as CD4+ or inducer T cells that consist of CD4+ as surface markers and recognizes a target cell via MHC Class II restriction. CD8 T cells perform diverse functions and differentiate into TH cells (TH 1, TH 2, TH 17) and Treg cells.
- TH 1 releases cytokines, interferons gamma and interleukins-2 that plays a vital role in the activation of macrophage and T-cells. It promotes cell-mediated immunity that destroys the cell infected with intracellular pathogens.
- TH 2 releases cytokines IL-4, 5 and 6 that primarily performs activation of B cells (producing antibodies).
- TH 17 releases cytokines IL-17 that provides immunity against chronic mucocutaneous infections.
- Treg cells are the regulatory T cells that release cytokine TGF beta, which functions to regulate the peripheral tolerance of neighbouring cells, immune response, and prevents autoimmune responses.
Therefore, the role of CD4 T cells involves activation of nearby immune cells (macrophages and NK cells), releasing cytokines, and helping B cells to stimulate antibodies production. It also monitors humoral and cellular immunity.
Memory T Cells
Memory T cells contain CD4+ or CD8+ as the surface markers. It provides long-lived memory and anamnestic secondary immune response in counter to the foreign body that formerly attacked the target cell.
T Cell Receptor
TCR or T cell receptor is a heterodimer that consists of glycoprotein chains arranged on the cell surface. The glycoprotein chain exists as two pairs either αβ or ƴδ TCR chains, which are attached via a disulphide bond. The TCR chains are held near the hinge region of the T cell membrane.
The transmembrane portion of the T cell receptor comprises positive charges, and also contains a short cytoplasmic tail. TCR includes variable, diversity, joining and constant encoded regions. Its structure resembles with the immunoglobulins and hence it also functions as antigen recognition unit.
T Cell Maturation
The haematopoietic stem cell of bone marrow releases multipotent T cell precursors, which will differentiate into progenitor T cells. The immature PRO T cells then travel to the thymus via the circulatory system for its maturation at the time of the embryonic or postnatal stage.
In the thymus, T lymphocytes undergo a maturation process, where the majority of thymocytes are eliminated and destructed in the thymus. The process here refers to the thymus selection process.
- When the receptors of thymocytes contact with the autoantigens, can survive in the thymus by receiving positive signals. In the thymus, all the T cells carry receptor molecules on its cell surface known as T cell receptors, specific for a particular antigen.
- When the TCRs contact with the self-antigens gets eliminated from the repertoire by receiving negative signals.
Then the T lymphocytes leave the thymus via blood circulation to the peripheral lymphoid organs. The T lymphocytes get activated once it encounters a specific antigen, and differentiate into an effector or regulatory T cells. The efficiency of the thymus to produce mature T cells involutes as the age increases, and a person becomes more susceptible to infections.
Therefore, we can conclude that the T lymphocytes are the small immune cells that perform distinct functions in our immune system. When a cell encounter with an antigen, the helper T cell stimulates the secretion of cytokines. The release of cytokines stimulates the antibody production by the plasma cells.
Regulatory T cells act to control immune reactions, and cytotoxic T cells (activated by various cytokines) kill the infected or tumorous cells. Memory T lymphocytes remember the infection site and provide an instant secondary response at a time of re-exposure of that antigen.