The Lymphatic System & Lymphoid Organs
Lymph vessels, lymph flow, lymph nodes, spleen, thymus, tonsils and MALT
By the end of this unit you can…
- ✓Describe the functions of the lymphatic system and the structure of lymphatic capillaries
- ✓Trace lymph from capillaries to the lymphatic ducts and venous blood
- ✓Name the lymphoid cells and tissues
- ✓Describe the structure and function of lymph nodes, spleen, thymus, tonsils and MALT
Key terms
Practice →1Lymphatic vessels & lymph
The lymphatic system has two parts: a network of lymphatic vessels that return fluid to the blood, and lymphoid tissues and organs that house immune cells. It (1) returns ~3 L/day of leaked fluid and plasma proteins to the bloodstream, maintaining blood volume, (2) absorbs dietary fats from the intestine (via lacteals → fatty chyle), and (3) houses lymphocytes for immune defense.
Lymphatic capillaries are blind-ended and remarkably permeable: overlapping endothelial cells form flap-like minivalves, and collagen anchoring filaments pull them open when interstitial fluid pressure rises. Proteins, cell debris, pathogens and cancer cells can easily enter — which is why infections and cancers spread through lymph. Once inside, the fluid is called lymph.
Lymph flows: lymphatic capillaries → collecting vessels (with many valves) → lymph nodes → lymphatic trunks → two ducts: the right lymphatic duct drains the right upper arm and right side of the head and thorax; the thoracic duct drains everything else, starting at the cisterna chyli. Both empty into the venous circulation at the junction of the internal jugular and subclavian veins.
Lymph flow is low pressure and relies on the same pumps as veins: skeletal muscle contraction, pressure changes during breathing, pulsations of nearby arteries, and contraction of smooth muscle in the trunk/duct walls. Valves prevent backflow.
2Lymphoid cells & tissue
| Cell | Role |
|---|---|
| T lymphocytes (T cells) | Manage the immune response; attack and destroy infected cells |
| B lymphocytes (B cells) | Produce plasma cells, which secrete antibodies |
| Macrophages | Phagocytize foreign substances; activate T cells |
| Dendritic cells | Capture antigens and bring them to lymph nodes to activate T cells |
| Reticular cells | Produce the reticular fiber stroma that supports the other cells |
Lymphoid tissue houses and provides a proliferation site for lymphocytes and a surveillance vantage point for lymphocytes and macrophages. It is mostly reticular connective tissue: diffuse lymphoid tissue (scattered cells in nearly every organ) and lymphoid follicles (nodules) — solid, spherical bodies with light-staining germinal centers where B cells proliferate.
3Lymph nodes
Hundreds of bean-shaped lymph nodes cluster along lymphatic vessels, especially in the cervical, axillary and inguinal regions. They (1) filter lymph — macrophages remove and destroy microorganisms and debris — and (2) help activate the immune system — lymphocytes encounter antigens and mount a response.
- Capsule
- Fibrous outer capsule; trabeculae extend inward dividing the node into compartments.
- Cortex
- Contains follicles with germinal centers (B cells); deeper cortex houses T cells.
- Medulla
- Medullary cords of lymphocytes and plasma cells; medullary sinuses with macrophages.
- Afferent vs efferent vessels
- Lymph enters via several afferent vessels on the convex side and leaves via fewer efferent vessels at the hilum — so lymph moves slowly through the node, giving time for filtering.
4Other lymphoid organs
The spleen — the largest lymphoid organ — sits in the left upper abdomen under the diaphragm. White pulp (lymphocytes around central arteries) does immune functions; red pulp (venous sinuses and splenic cords) removes old and defective RBCs and platelets and blood-borne pathogens. It also stores iron and platelets, and makes blood cells in the fetus.
The thymus, in the anterior mediastinum, is the site where T lymphocytes mature. It's large and active in childhood and gradually atrophies, replaced by fat and connective tissue in adults. Its cortex holds rapidly dividing T cells; the medulla has thymic corpuscles. Unlike other lymphoid organs, the thymus doesn't fight antigens directly — its stroma is epithelial cells, not reticular fibers, and a blood–thymus barrier keeps antigens out.
| Tonsil | Location |
|---|---|
| Palatine tonsils | Either side of the posterior end of the oral cavity (the ones usually removed) |
| Lingual tonsil | Base of the tongue |
| Pharyngeal tonsil (adenoids) | Posterior wall of the nasopharynx |
| Tubal tonsils | Around openings of the pharyngotympanic tubes |
Tonsils form a ring of lymphoid tissue at the entrance to the pharynx; their crypts trap bacteria and particles, letting immune cells build “memory”. MALT (mucosa-associated lymphoid tissue) protects the digestive, respiratory and genitourinary tracts — including Peyer's patches (aggregated lymphoid nodules in the wall of the distal small intestine) and lymphoid follicles in the appendix.