bursa of Fabricius

An epithelial and lymphoid organ that develops as a dorsal diverticulum of the proctodeal region of the cloaca in birds. The luminal (interior) surface of the bursa is plicated with as many as 15 primary and 7 secondary plicae or folds. These plicae have hundreds of bursal follicles containing follicle-associated epithelial cells, lymphocytes, macrophages, and plasma cells. Lymphoid stem cells migrate from the fetal liver to the bursa during ontogeny. In the bursa, these stem cells acquire the characteristics of mature, immunocompetent B cells[WP]. The bursa is an organ found in birds involved in B cell differentiation[GO].