Alterations of the thymus and other lymphoid tissue in young horses with combined immunodeficiency.

Combined immunodeficiency (CID) is a significant disease in terms of prevalence in Arabian foals and is a useful animal for study of a similar condition in children. Thymuses from all CID foals examined were extremely hypoplastic. Light and electron microscopic examination of thymuses from CID foals, as well as a thymus from an aborted CID fetus, demonstrate that the basic thymic structure is intact, despite a number of dissimilar morphologic appearances. From these data, we inferred that the thymic hypoplasia was caused by a failure of committed lymphocytes from the bone marrow to populate the organ. The lack of uniform organized lymphocytes in the spleen and lymph nodes provides considerable support for the absence of lymphoid precursors or their inability to respond to differentiating influences.