THE THYMUS

In vertebrates, a gland just above the heart, in the upper part of the mediastinum, behind the STERNUM and it extends upwards into the root of the neck. The human Thymus weighs about 10 to 15 grams at birth. It grows until puberty and then gradually shrinks. It’s maximum weight is around 30 - 40 grams (around 1 to 1.5 ounces). By the age of 40 it has returned to it’s weight at birth.

The Thymus consists of two lobes, each formed as an irregular branching framework of epithelial cells and lymphocytes. The lymphocytes, which originated from stem cells in red bone marrow, enter the Thymus and mature and develop into activated T-lymphocytes, i.e. ones able to respond to antigens (such as bacteria, viruses, allergens and incompatible proteins) encountered elsewhere in the body.

Some T-lymphocytes enter the blood, while others remain in the Thymus gland and are the source of future generations of T-lymphocytes.