The lymphoid system is intimately involved in immunological processes. The small lymphocyte that circulates through blood into lymphoid tissues, then through the lymph and back to the blood through the thoracic duct, is able to initiate immune responses after appropriate stimulation by antigen. However, the lymphocytes found in the thymus are deficient in this ability despite the fact that the thymus plays a central role in lymphocyte production and in ensuring the normal development of immunological faculty. During embryogenesis, lymphocytes are present in the thymus before they can be identified in the circulation and in other lymphoid tissues. They become "educated" in the thymus to recognize a great diversity of peptide antigens bound to the body's own marker antigen, the major histocompatibility complex, but they are purged if they strongly react against their own self-components. Lymphocytes differentiate to become various T cell subsets and then exit through the bloodstream to populate certain areas of the lymphoid system as peripheral T lymphocytes with distinct markers and immune functions.
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