Histologic sections of the thymus that reveal reactive follicles with germinal centers are diagnostic of

The thymus, derived from the third pair of pharyngeal pouches and inconsistently from the fouh pair, is divided into an outer coex and an inner medulla and is composed of lymphocytes and epithelial cells. The lymphocytes are mainly T cells, which are immature (thymocytes) in the coex and are mature in the medulla, where they have phenotypic characteristics of peripheral blood T lymphocytes. The epithelial cells are mainly located in the medulla, forming Hassall's corpuscles. The thymus normally has a few neuroendocrine cells, which may give rise to carcinoid tumors or small cell carcinoma, and a few myoid cells, which are similar to striated muscle cells and may play a role in the autoimmune pathogenesis of myasthenia gravis. The appearance of lymphoid follicles with germinal centers is diagnostic of thymic hyperplasia Reference: Robbins & Cotran Pathologic Basis of Disease, 9edition