Final answer:
The strict control of the number of T lymphocytes in both secondary lymphoid organs and circulation is known as T cell tolerance, a process essential for the immune system to prevent self-attack.
Step-by-step explanation:
The number of T lymphocytes, both in secondary lymphoid organs and in circulation, are kept under strict control, a process known as T cell tolerance. This process is essential in the immune system's ability to distinguish self from non-self, thus preventing an immune attack on the body's own cells. Thymic selection is a critical component of this process, where developing T cells, or thymocytes, undergo a series of positive and negative selections within the thymus gland. Through this selection, nearly 98% of thymocytes are eliminated, and the remaining 2% that exhibit tolerance to self-cells are allowed to mature and exit into the peripheral bloodstream. These mature naïve T cells eventually circulate to secondary lymphoid organs like the spleen, lymph nodes, and tonsils, where they are activated by the presentation of specific antigens by antigen-presenting cells (APCs) to mount an adaptive immune response.