Lymphoid Tissues
Learning objectives
Know the structure and function of the lymphoid tissues and organs: (including afferent and efferent flow and specialized vasculature)
- bone marrow
- thymus
- tonsil
- diffuse lymphoid tissue (BALT and MALT)
- lymph nodes
- spleen
Components of the Lymphatic System
The lymphoid system has primary lymphoid organs that produce lymphocytes and secondary lymphoid organs, which are the sites of immune response.
Primary lymphoid organs include bone marrow and thymus (encapsulated). All lymphocytes originate in the bone marrow. B cells (as well as monocytes, erythrocytes, granulocytes, and megakaryoctyes) remain in the bone marrow as they mature. T cells migrate to the thymus before maturation.
Secondary lymphoid organs include tonsil, lymph nodes, spleen, and diffuse lymphoid tissue (bronchial-associated lymphoid tissue or BALT in the lungs and mucosa-associated lymphoid tissue or MALT in the digestive tract).
Lymphatic vessels carry lymph fluid, which contains particulate matter, and protein that escape from blood capillaries as well as ingested fats, microorganisms, and other antigenic material that penetrate epithelial surfaces. Lymph is filtered in lymph nodes and then returned to the circulation via the thoracic duct and the right lymphatic duct.