Thymus and spleen anatomy, illustration. The thymus gland (upper centre) is part of the lymphoid system. It is located in the mediastinum area of the chest. Its role is to produce and mature T-lymphocytes (a specialised form of white blood cell), as well as other, more basic lymphocytes. It also controls the development of lymph nodes and the spleen. The spleen (lower right) is an abdominal organ that is a both a reservoir of blood, and part of the immune system. The spleen is a mixture of white and red pulp, storing red and white blood cells.