The Lymphatic System

The Lymphatic System

• Comprises lymphoid tissue and organs that initiate adaptive immune responses.
• Divided into primary (central) and secondary (peripheral) lymphoid organs.

Primary Lymphoid Organs

• Major sites of lymphocyte development, including the thymus and bone marrow (BM).

Secondary Lymphoid Organs

• Include the spleen, lymph nodes, and mucosal-associated lymphoid tissues (MALT), such as tonsils and Peyer's patches.
• Spleen responds to blood-borne antigens, lymph nodes respond to antigens from skin or external surfaces, and MALT protects mucosal surfaces (GI, GU, and respiratory tracts).

Lymph Nodes

• Composed of an outer cortex and inner medulla.
• Cortex contains an outer layer of B cells and an inner (paracortical) region of T cells and dendritic cells.
• B cells are organized into primary and secondary follicles, with secondary follicles containing germinal centers.

Spleen and MALT

• Organization of spleen and lymph nodes is similar.
• Spleen serves as a reservoir for platelets, RBCs, and granulocytes, and is the site of RBC destruction.

MALT

• Aggregates of non-encapsulated lymphoid tissue.
• Comprises B cell follicles and T cell-rich zones.

Lymphatic Vessels

• Lymph composition is identical to interstitial fluid.
• Interstitial fluid not reabsorbed into capillaries enters lymphatic capillaries, which drain back into the blood.
• Lymphatic vessels begin as close-ended lymphatic capillaries in spaces between cells.
• Lymphatic endothelial cells function as one-way valves, preventing backflow of lymph.

Lymphatic Capillary

• Lymphatic endothelial cells have anchoring filaments, which prevent backflow of lymph.

Cells of the Immune System

• Lymphocytes are small cells (8-10 µm diameter) with a large nucleus and thin rim of cytoplasm.

Lymphocytes

• Three main classes: B cells, T cells, and natural killer (NK) cells.

Natural Killer (NK) Cells

• Large granular lymphocytes or null cells, 13 µm diameter.
• Cytotoxic cells that kill tumor- and virus-infected cells, important part of innate immune response.

B Cells

• In birds, mature in the Bursa of Fabricus; in mammals, mature in bone marrow.
• On antigen recognition, B cells mature into antibody-secreting plasma cells, producing antibody (humoral response).

T Cells

• Initially develop in BM, complete maturation in thymus.
• B and T cells are spherical cells (7-9 µm diameter) with large nuclei and thin rim of cytoplasm.
• T cells mediate cellular immune response, on activation, differentiate into effector cells (Tc, Th1, Th2) that form the cell-mediated arm of the adaptive response.

The Spleen

• Organ weighing ~150 g, located behind the stomach, encapsulated by connective tissue fibers.
• Comprises mostly red pulp interspersed with white pulp, supplied by the splenic artery.
• White pulp is divided into T cell-rich areas (periarteriolar lymphoid sheath - PALS), primary and secondary lymphoid follicles, and a marginal zone of macrophages and lymphocytes.
• Red pulp consists of vascular sinusoids associated with RBCs, macrophages, dendritic cells, lymphocytes, and plasma cells, which end in venules that drain into the splenic vein.

Learn about the lymphatic system and its functions in immunity, including antigen presentation and adaptive responses, as well as its primary and secondary organs.