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Why are many developing T cells destroyed during selection in the thymus?
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lymphatic system
The human lymphatic system, showing the lymphatic vessels and lymphoid organs.
In the thymus, the vast majority of developing T cells (thymocytes) are destroyed during a rigorous selection process to ensure the immune system can effectively distinguish between "self" and "nonself" while preventing autoimmunity.12
This selection occurs in two primary stages:
- Positive Selection: Thymocytes are exposed to the body's major histocompatibility complex (MHC) molecules. Cells that are capable of recognizing these molecules are preserved, while those that cannot bind to them are destroyed because they would be unable to function in the immune response.2
- Negative Selection: Thymocytes that survive positive selection move to the medulla, where they are tested for their potential to attack the body’s own tissues. Those that demonstrate an ability to react against the body's own components are destroyed to prevent autoimmune reactions.12
Ultimately, this process is highly selective; only about 5 to 10 percent of developing thymocytes survive to exit the thymus and enter the lymphatic system and bloodstream.2