Unlike T lymphocytes, B lymphocytes arise from the bone marrow Bursa of Fabricus (in fowl) and gut-associated lymphoid tissue (Peyer's Patches); they are the major constituents of the humoral immunity or antibody-mediated immune system; they defend against viruses and bacteria that enter the blood and lymph; they are formed by the division of plasma cells; and plasma cells do not have inhibitory effect on the immune system.
