Immunology | Immune system| Immunity

              In humans and mice,bone marrow is the site of B-cell origin and development.Arising from lymphoid progenitors,immature B cells proliferate and differentiate within the bone marrow, and stromal cells within interact directly with B cells and secrete various cytokines that are required for development.


Bone marrow is not  the site of B-cell development in all species..In birds,a lymphoid organ called the bursa of fabricus,a lymphoid tissue associated with the gut,is the primary site of B-cell maturation,proliferation.In mammals such as primates and rodents,there is no bursa and no single counterpart to it as a primary lymphoid organ.In cattle and sheep,the primary lymphoid tissue hosting the maturation the maturation,proliferation,and diversification of B cells early in gestation is the fetal spleen.


Later in gestation,this function is assumed by a patch of tissue embedded in the wall of the intestine called the ileal Peyer's patch,which contains a large number of B cells.The  rabbit,too,uses gut-associated tissue suchas the appendix as primary lymphoid tissue for important steps in the proliferation and diversification of B cells.

The bone marrow is found within the central cavities of axial and long bones. It consists of hematopoietic tissue islands and adipose cells surrounded by vascular sinuses interspersed within a meshwork of trabecular bone. It accounts for approximately 3% of the body weight in adult rats , ~2% in dogs and ~5% in humans . The bone marrow is the major hematopoietic organ, and a primary lymphoid tissue, responsible for the production of erythrocytes, granulocytes, monocytes, lymphocytes and platelets.