The thymus is a small gland that is situated behind the top of the
breastbone in front of the trachea and plays an important role in
immunity. The thymus increases in size and activity until puberty then
begins to shrink. The thymus secretes several hormones which help
develop the immune system, one in particular called thymosin are
produced that stimulate the development of antibodies as well as
producing T-lymphocytes which are white blood cells that fight infection
and destroy any abnormal cells.