4-5 lymph

4. Lymph Nodes

a. Function

• Filter lymph to remove pathogens and debris.

• Site where lymphocytes are activated to respond to pathogens.

b. Structure

• Encapsulated with dense connective tissue.

• Divided into cortex and medulla.

• Cortex: Outer layer containing lymphoid follicles with germinal centers (B cell activation sites).

• Paracortex: Between cortex and medulla; contains T cells and dendritic cells.

• Medulla: Inner area with medullary cords containing B cells, plasma cells, macrophages.

c. Circulation

• Lymph enters node via multiple afferent lymphatic vessels.

• Flows through subcapsular sinus → cortical sinuses → medullary sinuses.

• Exits via fewer efferent lymphatic vessels at the hilum.

• Slow flow due to more afferent than efferent vessels allows effective filtration and immune activation.

5. Spleen

a. Functions

• Filters blood (not lymph).

• Removes aged or damaged RBCs and platelets.

• Stores platelets and monocytes.

• Initiates immune responses to blood-borne antigens.

• Site of fetal erythropoiesis.

b. Structure

• Largest lymphoid organ, enclosed by fibrous capsule.

• White pulp: Surrounds central arteries; contains lymphocytes and macrophages; site of immune response.

• Red pulp: Contains sinusoids filled with blood, splenic cords (reticular connective tissue, macrophages, RBCs); site of RBC breakdown.