The Thymus is a ductless two-lobed gland lying behind the breast bone and composed mainly of lymphoid cells. It plays a part in setting up the body's immunity system. After puberty, it declines in size.
Autoimmune difficulties are thought to be caused by an improperly functioning thymus. A type of cancer called Lymphoma can arise from the thymus gland
The New American Desk Encyclopedia, Penguin Group, 1989