THE THYMUS: EXPERIMENTAL PHYSIOLOGY,AND PATHOLOGY IN MAN

The thymus is an integral part of the immunological system. It is a site of intense lymphopoiesis, especially in early life. Neonatal thymectomy in mice causes runting and death due to gross immunological deficiencies. These deficiencies are determined by lymphopenia, and by lack of a lymphotrophic hormone secreted by the epithelial cells of the medulla; this hormone confers on lymphocytes the capacity to respond to antigenic stimulation. The thymus may be the main source of lymphoid cells carrying new or primary patterns of immune reactivity; it is thus “first-level” or “central” lymphoid tissue, which seeds cells to “second-level” or “peripheral” lymphoid tissues in the lymph nodes and spleen. Pathological lesions of the thymus in man include aplasia, hyperplasia, dysplasia and neoplasia. Gross aplasia characterizes the immunological deficiency diseases of infancy, including the lymphopenic type of congenital agammaglobulinæmia. Hyperplasia accompanies thyrotoxicosis. Dysplasia refers to the lymph follicle-germinal centre development in myasthenia gravis, probably an autoimmune disease, and to the proliferation in the medulla of spindle-epithelial cells in lupus erythematosus, an autoimmune disease. Neoplasia occurs as benign thymoma, which may be accompanied by extrathymic diseases which are possibly autoimmune in origin; these include myasthenia gravis, red cell aplasia, polymyositis, agammaglobulinæmia and lupus erythematosus. These diseases may in some way be caused by the thymoma; alternatively, the thymoma may represent the result of continuing hyperplasia of the thymus provoked by a primary autoimmune process. The place of thymectomy in the treatment of autoimmune disease is discussed. It is an established procedure in myasthenia gravis, and has been successful in two cases of autoimmune hæmolytic anæmia in infancy. We review our experience with thymectomy for three patients with systemic lupus erythematosus.