Bursa of Fabricus is primary lymphoid organ in birds. Differentiation, maturation and proliferation of B Lymphocytes take place in this organ. B Lymphocytes are responsible for the production of antibodies and humoral immunity. Bursa is seen only in birds and it is named after the person Fabricus, who invented the organ in birds. Like thymus, bursa of Fabricus is an epithelial structure developed as epithelial diverticulum from the dorsal region of cloaca.
Development of this organ in birds takes place on about 15th day of development and by the time of hatching it develops into a fully functional organ. It attains maximum size (about 3 cm in diameter) in 4 months of age; later like thymus it also shows atrophy.
Similar to the development of thymus, epithelial diverticulum from cloaca region is colonized by pleuripotent stem cells to undergo proliferation and differentiation to become immune competent lymphocytes known as “B cells” (B stands for bursa).
Histological study of bursa shows similar characteristic features of thymus. Like thymus, bursa is divided into follicles by connective tissue and each follicle has cortex and medulla regions. The differentiated B cells show morphological similarity with T lymphocytes, but show significant difference in their surface markers and function.
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The mature and immune competent B cells leave the primary lymphoid organ to reach secondary lymphoid organs through blood circulation. In secondary lymphoid organs the immune competent B lymphocytes produce antibodies in response to antigenic stimulus to provide humoral immunity.
In mammals and other vertebrates, bursa of Fabricus is absent, but certain regions of bone marrow serve as the bursa equivalents for differentiation and production of B lymphocytes. In early embryological stages spleen and ileal Peyer’s patch act as primary lymphoid organ for differentiation, maturation and proliferation of B cells.
In bone morrow, the stromal cells influence the differentiation and maturation process through their cytokines.