The letter "T" designates from its site of maturation, in the "Thymus". T-cells regulate the "Cell-mediated immunity". The cells operates against cells bearing intracellular organisms, T-cells recognize antigen only when it is on the surface of a body cells. T-cells have receptors on their membrane surface (cell markers), which belongs to an important group of molecules known as the “Major Histocompatibility Complex (MHC)”.
There are 4 types of t-lymphocytes are present in immune system. They are :
i) T-Helper cells (TH-Cells):
- The TH-cells recognize and bind to the combination of the antigen with the class -II MHC on the macrophage surface.
- TH-cells produce a variety of soluble factors known as "Cytokines". The cytokines induce gamma-interferon" and other "Macrophage activating factors".
- The cytokines restore the microbicidal mechanism of the macrophage and bring about the death of the intracellular microorganisms.
II) Cytotoxic T-Cells (Tc -Cells):
- Each Tc recognizes antigen only association with a cell marker, the class-l MHC.
- Tc also releases gamma-interferon.
- The cells recognize virally infected cells, which are killed before the virus replicates.
- Like B-cells, Tc - cells require help from Th-cells.
- These cells attack infected cells and cancer cells.
- The cells discharge a protein called "Perforin", which lyses the infected cell.
iii) T-Suppressor cells (Ts-cells):
- The Ts-cells promote immune response.
- The cells suppress a specific antibody formation (Infectious tolerance).
- The "Infectious tolerance" phenomenon was shown that mice could be made unresponsive by infection of a heavy dose of sheep RBC into them; their T-cells suppressed specific antibody formation in the recipient mice.
- The T-cell receptor was elucidated through the powerful recombinant DMA technology.
- The T-cell receptor called "Ti" (which is a made up of two non-identical polypeptides- α & β chains and joined by a disulfide bond).
- The Ti associated with another protein called "CD3".
Ti +CD3 –> Ti-CD3 complex
(T-cell receptor complex)
- After many years, the immunologists identified second type of receptor composed of gamma and 6-chains.
- The CDs is made up of at least three polypeptide chains (γ,δ and Є).
iv) Ts-lymphocytes: (T suppressor Cells):
- Ts cells are a sub population of T cells that suppresses the activity of B-cells and other T-cells.
- They are the regulatory T cells.
- They inhibit antibody production by B cells.
- They suppress the functions of the T killer cells and T helper cells.
Development of T-lymphocytes:
The T-lymphocytes are derived from "Hemopoietic stem cells" of bone marrow. Some of these cells turn into "Lymphopoietic progenitor". The lymphoid progenitor cells turn into "Pre-T-lymphocytes". The Pre-T-cells leave the bone marrow and enter the thymus. In the thymus they mature into T-lymphocytes through the influence of thymic hormones. The mature T-lymphocytes enter the blood and colonies the secondary lymphoid organs such as spleen, lymph nodes, payer’s patches, etc. Here they become functional on exposure to antigens.