 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.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.