lymph node

Lymph nodes are encapsulated by a fibrous capsule from which trabeculae extend inward. The outer cortex contains follicles (B cell zones) with germinal centers during immune responses. The inner paracortex is T cell-rich. The medulla contains medullary cords (antibody-secreting plasma cells) and sinuses draining to the efferent lymphatic vessel at the hilum.

Lymph enters lymph nodes through multiple afferent lymphatic vessels and percolates through a network of subcapsular, cortical, and medullary sinuses. Macrophages lining these sinuses phagocytose pathogens, cell debris, and foreign particles, while lymphocytes sample antigens to initiate adaptive immune responses.

Lymph node enlargement (lymphadenopathy) occurs when lymphocytes rapidly proliferate in response to antigens, filling lymphoid follicles with activated B and T cells. Increased macrophage activity and edema within the node also contribute to swelling, which clinically signals ongoing immune activity or infection.