The thymus is a small gland comprised of two lobes located in front of the heart and behind the sternum. It plays a major role in the immune system by transforming undifferentiated white blood cells into disease specific T cells. It's at its most active before the onset of puberty and afterwards it slowly starts turning into fat. However, it continues to produce T cells throughout a person's life.
The ancient Greeks first described the gland, and the Greek physician Galen was the first person to describe how it atrophied as a person aged. However, it was often thought of as a vestigial organ (essentially, where white blood cells went to die) until 1961 when Jacques Miller did experiments on rats that showed it was tied to the production of what he named T cells after the organ.
Congenital deficiencies in the thymus can lead to immunosuppression disorders. However, a tumor of the gland, a thymoma, can also lead to autoimmune disorders such as myasthenia gravis. Some lymphomas can also originate in the thymus.
The thymoma can be removed without tremendous adverse effects as the T cells, once formed, are very long lived in the body. It's most commonly removed to gain access to the heart during heart surgery.
The thymus (center) from Gray's Anatomy