Thymocytes are hematopoietic progenitor cells that mature into T cells in the thymus. Through thymopoiesis, defective cells are eliminated, and successful cells enter the bloodstream as mature T cells. The process includes beta selection, positive selection, and negative selection, which eliminates thymocytes capable of binding to self proteins. Failure of regulatory cells can lead to autoimmune diseases.
Thymocytes are cells that live in the thymus, an organ of the immune system. Biologists classify these cells as hematopoietic progenitor cells, which means they are capable of differentiating into other blood cells. Within the thymus, thymocytes enter a process of selection and maturation called thymopoiesis and become T lymphocytes or T cells, cells important to the immune system. During the three stages of thymopoeia, cells that are defective or harmful to the body are filtered out and eliminated. If a thymocyte passes all three stages, it enters the body’s normal bloodstream as a mature T cell.
Hematopoietic progenitor cells in the bone marrow that travel through the blood and naturally reach the thymus automatically become thymocytes. In the first stage of thymopoiesis, the process of beta selection, an early thymocyte attempts to make a T-cell receptor by cutting its DNA and linking its different gene fragments. In this way, each T cell has a different T cell receptor that can recognize and defend against a wider variety of bacteria and viruses. The body eliminates any thymocytes that are unable to successfully display their T-cell receptors on their cell surfaces.
In the second step of thymopoeisis, the process of positive selection, a thymocyte must demonstrate that its T-cell receptor can bind with a major histocompatibility complex (MHC) molecule. When these molecules have proteins on their cell surface, a T cell must bind to them to determine whether the protein is safe or dangerous for the body. All thymocytes with T cell receptors that are unable to bind to MHC molecules undergo apoptosis, cell death. Some thymocytes at risk of apoptosis can save themselves by fashioning new T-cell receptors during this phase.
Thymopoeia ends when thymocytes pass the process of negative selection, during which the body eliminates all thymocytes capable of binding to self proteins. Autoproteins are harmless proteins made by the body, and T cells that bind to autoproteins can accidentally trigger immunological responses. After successfully undergoing thymopoesis, the cells enter the bloodstream as mature T cells and participate as members of the immune system. Some T cells can pass the process of negative selection even though they can bind to autoproteins, but these cells are usually suppressed or eliminated by regulatory cells. If these regulatory cells fail, the person can develop an autoimmune disease in which the cells attack the body.
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