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How does the thymus remove self-reactive T cells?

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immune system disorder False-color scanning electron micrograph of a T cell infected with HIV (human immunodeficiency virus), the agent that causes AIDS (acquired immunodeficiency syndrome).

The thymus removes self-reactive T cells through a process known as negative selection.1

During T cell development, thymocytes (developing T cells) first undergo positive selection in the thymic cortex, where they are tested for their ability to recognize the body's major histocompatibility complex (MHC) molecules. Those that successfully bind are preserved, while those that cannot are destroyed.1

The surviving thymocytes then migrate to the medulla of the thymus. In this region, they undergo negative selection, where cells that demonstrate the ability to attack the body’s own tissues are identified and destroyed. This mechanism is a vital component of central tolerance, ensuring that only a small fraction—approximately 5 to 10 percent—of produced T cells survive to exit the thymus and enter the bloodstream. Any self-reactive cells that escape this process and reach peripheral tissues are subsequently managed by peripheral immune tolerance mechanisms, such as anergy, suppression by regulatory T cells, or deletion via apoptosis.2134