Overview
This lecture explains the structure and function of the lymphatic system and its key role in immune system organization, focusing on lymph nodes, spleen, thymus, and mucosa-associated lymphoid tissues (MALT).
Why the Lymphatic System is Needed
- Immune cells need to meet and exchange activating and co-stimulatory signals to respond to threats efficiently.
- The lymphatic system serves as a transportation and meeting network for immune cells throughout the body.
Components of the Lymphatic System
- Lymphatic vessels collect lymph (interstitial fluid) from tissues and return it to the bloodstream.
- Lymph nodes are distributed throughout the body to filter lymph and coordinate immune responses.
- Spleen filters blood, removes old red blood cells, and organizes immune cells for blood-borne threats.
- Thymus is where T cells develop and mature.
- Mucosa-associated lymphoid tissues (MALT) protect mucosal surfaces and regulate immune responses to non-threatening antigens.
Lymph Node Structure and Function
- Lymph nodes have a connective tissue capsule and internal sinuses lined with macrophages and dendritic cells.
- Organized into B cell zones (outer cortex/follicles), T cell zones (deeper cortex), and helper T cell-rich germinal centers.
- Dendritic cells present antigens to T cells; activated lymphocytes divide and migrate to infection sites.
Spleen Organization and Function
- Red pulp (75% of spleen) removes damaged red blood cells and recycles iron.
- White pulp contains T cell zones (PALS) and B cell follicles, separated by the marginal zone with specialized B cells.
- Only filters blood, not lymph; also reserves blood, platelets, and monocytes.
Thymus and T Cell Development
- Located above the heart, most active during childhood.
- T cell progenitors mature in cortex (immature) and medulla (mature) regions before entering circulation.
Mucosa-Associated Lymphoid Tissues (MALT)
- Found in digestive, respiratory, and urogenital tracts, and other mucosal surfaces.
- Specialized structures (e.g., Peyer’s patches in gut, NALT/BALT in airways) sample antigens via M cells.
- Produces IgA antibodies and regulates immune response to avoid overreacting to harmless antigens.
Key Terms & Definitions
- Antigen — foreign molecule recognized by immune cells.
- Cognate antigen — specific antigen recognized by a unique lymphocyte.
- Lymph — fluid collected from tissues containing waste, debris, and pathogens.
- Lymph node — organ that filters lymph and coordinates immune cell interactions.
- Spleen — organ that filters blood and coordinates immune responses to blood-borne pathogens.
- Thymus — site of T cell development and maturation.
- MALT — mucosa-associated lymphoid tissue that protects mucosal surfaces.
- PALS — periarteriolar lymphoid sheath, T cell zone in spleen.
- IgA — antibody type produced at mucosal surfaces.
Action Items / Next Steps
- Review immune cell types and their functions.
- Prepare to learn about immune cell communication in the next lecture.