THE LONG AND SHORT OF IT

CERTAIN parts of your body never stop growing, while others shrink with age. This week, the thymus. THE thymus is a large gland just under the breast-bone, close to the heart. During childhood and adolescence it plays a role in boosting immunity — it secretes the hormone thymosin, which produces T-cells, a type of white blood cell that protects the body from viruses and infections. By the time we reach puberty, the gland measures 3-4 cm across (around the size of a kiwi fruit) and weighs about 35 g.

By adulthood, the thymus has trained the body to produce all the immune cells it needs. Now surplus to requirements, it begins to waste away and becomes replaced with fatty tissue.

The gland withers so much that by the age of 70 it is the size of a small marble and measures only about 1 cm across.