T lymphocytes (T cells, named after the thymus-dependent development) are major components of the adaptive immune system. It is directly involved in destroying infected host cells, producing cytokines, activating other immune cells and in regulating immune response.
These lymphocytes have a central role in immune response. Due to the presence of t-cell receptors on the surface of cells, they are distinguishable from other lymphocytes. Origination of these cells is in the form of precursor cells from the bone marrow further goes on to grow into different types of t cells, once they arrive at the thymus gland. Despite leaving the thymus, T cells continue to differentiate.
T Cell Production
T cells arise from heamotopoeitic stem cells synthesised in the bone marrow. Few multipotent cells turn progenitor cells leaving the bone marrow and travelling to the thymus through the blood. These cells mature in the thymus.
In the thymus, the T cells experience a type of selection phenomena which most of the developing T cells (known as thymocytes) do not successfully pass and hence succumb. However, those Thymocytes which pass, interact with the self-MHC molecules accept positive signals to survive and the Thymocytes possessing receptors obtain negative signals (those having self-antigenic properties) are eliminated from the repertoire.
T cells then develop their own T cell receptor (TCR) and are antigen-specific. Those T cells surviving thymic selection mature and exit the thymus. They then pass through the peripheral lymphoid structures to take on a particular antigen and get stimulated. Upon activation, T cells grow and differentiate into an effector T cell.
With ageing, the thymus involutes consequently we generate few naive T cells. This corresponds to saying that older bodies produce reduced T cell diversity contributing to the increased susceptibility to infections with ageing.
T Cells – Types
T cells that have not interacted with specific antigens are naive T cells. These cells can interact with ACPs (antigen presenting cells) in the peripheral lymphoid organs. These ACPs make use of an MHC molecule to present the antigen. Upon specific-antigen recognition, T cells grow and differentiate into effector T cells of a specific category. Such cells interact with host cells to perform their roles.
There exist proteins to differentiate main groups of effector T cells – CD4 or CD8 cells, which are used either to bind or as co-receptors. When naive T cells interact with CD4 cells they turn T helper cells, and with CD8 cells, they turn cytotoxic T cells, performing their own activities.
T helper cells or Th have a broad range of functionality compared to CD8 cells. Consequently they can differentiate into several subclasses such as Th1, Th2, Th17 and regulatory T cells. Presence of peptide antigens by MHC Class II molecules activates these cells. Expression of these cells is on the surface of APCs. Such molecules act with protein known as CD4 on the T helper cells assisting this type of cell.
Primarily, CD4 cells include activation of other immune cells which release cytokines and help B cells to produce antibodies. It is involved in shaping, activating and regulating the adaptive immune responses.
Target cells are killed by the cytotoxic T cells. It basically does so by liberating cytotoxic granules to be killed into the cell. These cells detect their specific antigen when offered by the MHC Class I molecules found on the surface of all nucleated cells. These molecules interact with a protein known as CD8 cells helping identification of cell type. These cells need to be signalled from other cells for their activation. Cells from whom signalling is expected are CD4 cells or dendritic cells. Primarily, they are involved in killing infected cells, tumorous cells and cells with intracellular bacteria.
Memory T cells
The main role of these cells is rendering memory to the immune system against antigens that are encountered before. These cells can be CD8+ or CD4+ cells. Whenever there is an infection, memory T cells are formed. These cells are long-lived and specific to antigens. These cells are important as they can immediately expand to significant numbers of effector T cells on exposure in future to the antigen. They have a low threshold for activation.
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