Thymocytes are cells that live in the thymus, an organ of the immune system. Biologists categorize these cells as hematopoietic progenitor cells, which means that they are able to differentiate into other blood cells. Within the thymus, thymocytes enter a selection and maturation process called thymopoiesis and become T lymphocytes or T cells, important cells for the immune system. During the three stages of thymopoeisis, defective cells or cells that are harmful to the body are filtered out and eliminated. If a thymocyte passes all three stages, it enters the body’s normal blood circulation as a mature T cell.
The hematopoeitic 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 beta selection process, an early thymocyte attempts to create a T cell receptor by cutting up DNA and linking its different gene fragments. 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 not able to successfully show their T cell receptors on their cell surfaces.
In the second stage of thymopoeisis, the positive selection process, a thymocyte must prove that its T cell receptor can bind with a major histocompatibility complex (MHC) molecule. When these molecules have proteins on their cell surfaces, a T cell must bind with them to determine whether the protein is safe or dangerous to the body. Any 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 stage.
Thymopoeisis ends when thymocytes pass the the negative selection process, during which the body eliminates any thymocytes that are able to bond to self-proteins. Self-proteins are harmless proteins made by the body, and T cells that bond to self-proteins can accidentally trigger immunological responses. After successfully passing thymopoesis, cells enter the blood stream as mature T cells and participate as members of the immune system. Some T cells may make it through the negative selection process even though they can bond to self-proteins, but these cells are usually suppressed or eliminated by regulatory cells. If these regulatory cells fail, the person may develop an autoimmune disease in which the cells attack the body.