The thymus is an organ of the lymphatic system involved in development of T cells. In humans it lies in the chest behind the sternum. Its anatomical position and its contribution to immunity are different properties: locating an organ does not by itself reveal its function.[ref]
Thymus in the NCI Dictionary of Cancer Terms. Definition: lymphatic organ, T lymphocytes and position behind the breastbone
A motivating experiment removed the thymus from newborn mice. In his firsthand account, Jacques Miller describes finding a deficiency of lymphocytes in other lymphoid tissues and failure to reject foreign skin grafts. Supplying circulating lymphocytes or a thymus graft could restore the graft-rejection response. Removal, a functional test and rescue together connected the thymus with development of immune competence.[ref]
Professor Jacques Miller, pathologist in Conversations with Australian scientists. Why did the mice waste away? An unexpected finding: lymphocyte deficiency, graft acceptance and rescue
A further comparison separated two consequences. In germ-free conditions, mice subjected to neonatal thymectomy could remain healthy yet still fail to reject skin grafts. Thus wasting illness and the measured graft-rejection defect were not interchangeable observations.[ref]
For an original experimental-design outline, compare sham-operated and thymectomized animals under matched conditions, then score lymphocyte abundance, health and graft rejection separately. Include a defined rescue condition. These endpoints distinguish loss of immune competence from a nonspecific effect of surgery. The neonatal timing matters: an experiment on development does not justify assuming that removal at every age has identical effects.
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