Biology · Chemical Coordination and Integration · NEET
It is both. In Chemical Coordination (Ch 19) it is an endocrine gland because it secretes the hormone thymosin. In Immunity (Ch 7) it is a primary lymphoid organ where T-lymphocytes mature. NEET can ask from either angle, so remember both roles. Bone marrow and thymus are the two PRIMARY lymphoid organs.
Both, but the main answer is cell-mediated immunity. Thymosins play a major role in the differentiation (maturation) of T-lymphocytes, which provide cell-mediated immunity. NCERT adds that thymosins also promote the production of antibodies to give humoral immunity. So if the question forces one answer, choose cell-mediated immunity.
The thymus is large at birth and keeps reducing in size, becoming very small after puberty. This age-related shrinking is called involution. Because it degenerates in old people, thymosin production falls, so the immune response of old persons becomes weak. This 'weak immunity in old age' link is a favourite NEET point.
Thymosin is a PEPTIDE (protein) hormone, not a steroid. NCERT clearly calls thymosins 'peptide hormones'. This matters because peptide hormones act through membrane receptors, while steroid hormones (like cortisol, sex hormones) enter the cell.
The thymus is a lobular structure located between the lungs, behind the sternum (breastbone), on the ventral (front) side of the aorta. In immunity chapter NCERT describes it as a lobed organ near the heart, beneath the breastbone. Same organ, two descriptions.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The thymus secretes peptide hormones called thymosins.
Thymosin drives the differentiation (maturation) of T-lymphocytes, which provide cell-mediated immunity. It also promotes antibody production for humoral immunity.
It is a PRIMARY lymphoid organ, along with bone marrow. This is where immature lymphocytes (T-cells) mature. Spleen, lymph nodes, tonsils and Peyer's patches are secondary lymphoid organs.
The thymus degenerates (shrinks) with age, so it makes less thymosin. Lower thymosin means fewer well-trained T-lymphocytes, so the immune response of old people becomes weak.
Thymosin is a peptide hormone.
