Thymus - Definition

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The thymus is an organ located in the upper section of the thorax, behind the sternum. It is in this organ where certain cells produced by the bone marrow develop, notably those which are involved in the body's defense system, such as T-lymphocytes (T for thymus, distinct from lymphocytes that develop in other organs, such as B-lymphocytes). They are developed, or "educated", by way of the thymic hormones secreted by this organ, notably including thymulin. This organ is very active in a baby's development, vastly reduces in size, called involution, during the child's 2nd year.