Definition of thymus. : a glandular structure of largely lymphoid tissue that functions especially in cell-mediated immunity by being the site where T cells develop, is present in the young of most vertebrates typically in the upper anterior chest or at the base of the neck, and gradually decreases in size and activity after puberty.
1 The thymus gland is very active from before birth until puberty, and it functions as both a lymphatic organ and an endocrine organ (an organ of the endocrine system that produces hormones). In order to understand the role the thymus gland plays in immunity, it’s helpful to first distinguish between T lymphocytes and B lymphocytes.
The T cell’s cytotoxicity comes from the cytokines it produces. Despite the thymus’ essential role in immune health, the thymus gland is not active during our entire lifetime. In fact, it is only active until puberty and becomes non-functional in adulthood.