A mass of glandular
tissue located in the neck or chest of most vertebrates. Found in the upper chest under the breastbone in humans, the thymus is essential to the development of the body's system of
immunity beginning in fetal life (i.e., before birth). The thymus processes white blood cells, known as lymphocytes, which kill foreign cells and stimulate other immune cells to produce
antibodies. The gland grows throughout childhood until puberty and then gradually decreases in size. See also
Antibodies;
Lymphocyte.