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T cells are one of the important types of white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell receptor (TCR) on their cell surface. T cells are born from hematopoietic stem cells, [1] found in the bone marrow.
Thymocytes are classified into a number of distinct maturational stages based on the expression of cell surface markers. The earliest thymocyte stage is the double negative stage (negative for both CD4 and CD8), which more recently has been better described as Lineage-negative, and which can be divided into four substages.
Once mature, T cells emigrate from the thymus to provide vital functions in the immune system. [11] [12] Each T cell has a distinct T cell receptor, suited to a specific substance, called an antigen. [12] Most T cell receptors bind to the major histocompatibility complex on cells of the body.
T cells are formed in bone marrow and then migrate to the cortex of the thymus to undergo maturation in an antigen-free environment for about one week where a mere 2–4% of the T cells succeed. The remaining 96–98% of T cells die by apoptosis and are phagocytosed by macrophages in the thymus.
In order for lymphocytes such as T cells to become immunocompetent, which refers to the ability of lymphocyte cell receptors to recognize MHC molecules, they must undergo positive selection. [ 1 ] Adaptive immunocompetence is regulated by growth hormone (GH), prolactin (PRL), and vasopressin (VP) – hormones secreted by the pituitary gland.
A lymphocyte is a type of white blood cell (leukocyte) in the immune system of most vertebrates. [1] Lymphocytes include T cells (for cell-mediated and cytotoxic adaptive immunity), B cells (for humoral, antibody-driven adaptive immunity), [2] [3] and innate lymphoid cells (ILCs; "innate T cell-like" cells involved in mucosal immunity and homeostasis), of which natural killer cells are an ...
T-cell function decline begins with the progressive involution of the thymus, which is the organ essential for T-cell maturation. This decline in turn reduces IL-2 production [28] [29] and reduction/exhaustion on the number of thymocytes (i.e. immature T cells), thus reducing peripheral naïve T cell output.
RANKL also has a function in the immune system, where it is expressed by T helper cells and is thought to be involved in dendritic cell maturation. It is a dendritic cell survival factor and helps regulate T cell-dependent immune responses. T cell activation induces RANKL expression and can lead to an increase of osteoclastogenesis and bone loss.