Overview
This lecture covers the cells, tissues, and organs of the lymphatic system, focusing on their roles in immunity and protection against foreign substances.
Cells of the Lymphatic System
- The lymphatic system contains three main cell types: reticular cells, macrophages, and lymphocytes.
- Reticular cells produce reticular connective tissue, the single tissue type of the lymphatic system.
- Macrophages phagocytose (engulf and digest) foreign substances and activate T cells.
- Lymphocytes are divided into T cells (kill foreign cells using chemicals/enzymes) and B cells (produce antibodies to mark invaders).
Lymphatic Tissues
- Reticular connective tissue, made by reticular cells, houses macrophages and lymphocytes.
- Lymphatic tissue can be diffuse (loosely arranged, present in many organs) or organized into lymphoid follicles (tightly packed with a B cell center and T cell surround).
- Lymphoid follicles can exist independently or within organs like lymph nodes.
Lymphatic Organs
- Primary lymphoid organs: red bone marrow (origin/maturation of B cells; origin of T cells) and thymus gland (maturation of T cells).
- Secondary lymphoid organs: sites where lymphocytes encounter antigens (lymph nodes, spleen, MALT tissues).
Spleen
- Located under the left ribcage, encapsulated by fibrous tissue.
- Contains white pulp (clusters of lymphocytes on reticular fibers) and red pulp (breaks down old red blood cells).
- White pulp is active in immune responses; red pulp recycles blood cell components.
Mucosa Associated Lymphoid Tissue (MALT)
- MALT is found in mucous membranes of the digestive, respiratory, urinary, and reproductive systems.
- Function is to guard against environmental pathogens at mucosal surfaces.
Major MALT Structures
- Tonsils: ring of lymphoid tissue in the throat, with crypts and follicles to trap and destroy pathogens.
- Peyer's patches: lymphoid follicles in the small intestine wall, providing a barrier against bacteria in food.
- Appendix: outpouching at the junction of small and large intestines, lined with lymphoid follicles to combat trapped bacteria.
Key Terms & Definitions
- Reticular cell — produces reticular connective tissue.
- Macrophage — phagocytic immune cell that engulfs and digests foreign substances.
- Lymphocyte — white blood cell; includes T cells (killers) and B cells (antibody producers).
- Reticular connective tissue — network that supports lymphatic cells.
- Lymphoid follicle — dense cluster of lymphocytes, B cells in the center, T cells outside.
- Primary lymphoid organ — site of immune cell origin/maturation (bone marrow, thymus).
- Secondary lymphoid organ — site where immune response is initiated (lymph node, spleen, MALT).
- White pulp — lymphocyte-rich area in spleen.
- Red pulp — area in spleen that recycles old red blood cells.
- MALT — mucosa-associated lymphoid tissue, protects mucosal surfaces.
Action Items / Next Steps
- Review and answer lecture questions on lymphatic cell types, tissues, and organs.
- Prepare for final video on the structure and function of lymph nodes.