<P> Galen was the first to note that the size of the organ changed over the duration of a person's life . </P> <P> In the nineteenth century, a condition was identified as status thymicolymphaticus defined by an increase in lymphoid tissue and an enlarged thymus . It was thought to be a cause of sudden infant death syndrome but is now an obsolete term . </P> <P> Due to the large numbers of apoptotic lymphocytes, the thymus was originally dismissed as a "lymphocyte graveyard", without functional importance . The importance of the thymus in the immune system was discovered in 1961 by Jacques Miller, by surgically removing the thymus from one day old mice, and observing the subsequent deficiency in a lymphocyte population, subsequently named T cells after the organ of their origin . Recently, advances in immunology have allowed the function of the thymus in T - cell maturation to be more fully understood . </P> <P> The thymus is present in all jawed vertebrates, where it undergoes the same shrinkage with age and plays the same immunological function as in human beings . Recently, a discrete thymus - like lympho - epithelial structure, termed the thymoid, was discovered in the gills of larval lampreys . Hagfish possess a protothymus associated with the pharyngeal velar muscles, which is responsible for a variety of immune responses . Little is known about the immune mechanisms of tunicates or of Amphioxus . </P>

What does the thymus do in the body