Summary
Highlights
The lymphatic system, interconnected with the circulatory system, plays a crucial role in immune function. Immune cells, spread throughout the body, recognize antigens (foreign molecules) and require co-stimulatory signals for activation. B and T lymphocytes have specific antigen recognition, and their activation often involves interaction with other cells, which is facilitated by the lymphatic system.
The lymphatic system is essential for immune cells to meet and activate each other. Lymph, containing waste products and pathogens, is filtered through lymph nodes. These nodes serve as strategic outposts where immune cells (B cells, T cells, dendritic cells, macrophages) can interact, receive activation signals, and initiate an immune response. Lymph nodes are highly organized with specific compartments for different cell types.
Each lymph node is encapsulated and receives lymph through afferent vessels into sinuses, which are lined with macrophages and dendritic cells. The outer cortex is the B cell zone, containing follicles, while the deeper T cell zone is rich in T cells. Dendritic cells present antigens to T cells, and helper T cells facilitate B cell activation in germinal centers. Naïve lymphocytes circulate through lymph nodes until they find their cognate antigen and become activated.
The spleen, the largest lymphatic organ, has two main functions: removing old red blood cells in the red pulp and filtering pathogens and immune complexes from the blood in the white pulp. Unlike lymph nodes, the spleen only filters blood. The red pulp contains Cords of Billroth and venous sinuses where old red blood cells are disposed of by macrophages. The white pulp resembles lymph nodes with T cell zones (periarteriolar lymphoid sheath) and B cell follicles.
The thymus is a crucial lymphoid organ for T cell development, most active during infancy and childhood. T cell progenitors migrate from the bone marrow to the thymus, where thymic stromal and epithelial cells guide their maturation in distinct cortical and medullary regions. Once mature, naïve T cells leave the thymus to circulate throughout the body.
MALT refers to lymphoid tissues found in mucosal surfaces like the digestive, respiratory, and urogenital tracts. These surfaces are permeable and susceptible to infection, so MALT provides localized immune defense. MALT structures are similar to lymph nodes with B cell follicles and T cell zones, but have specialized features. For example, M cells in GALT (Peyer’s patches) and NALT/BALT sample antigens. MALT also regulates immune responses to prevent overreactions to harmless antigens, frequently producing IgA antibodies.