Thymus extracts are extracts derived from the thymus glands usually of young calves (bovine). The thymus is one of our major immune system glands. It is composed of two soft pinkish-gray lobes lying in bib-like fashion just below the thyroid gland and above the heart. To a large extent, the health of the thymus determines the health of the immune system. The thymus is responsible for many immune system functions, including the production of T lymphocytes, a type of white blood cell responsible for "cell-mediated immunity." Cell-mediated immunity refers to immune mechanisms not controlled or mediated by antibodies. Cell-mediated immunity is extremely important in the resistance to infection by certain bacteria, yeast (including Candida albicans), fungi, parasites, and viruses (including herpes simplex, Epstein-Barr, and the viruses that cause hepatitis). Cell-mediated immunity is also critical in protecting against the development of cancer, allergies, and autoimmune disorders such as rheumatoid arthritis. The thymus gland also releases several hormones, such as thymosin, thymopoeitin, and serum thymic factor, that regulate many immune functions.