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6. MALT (Mucosa-Associated Lymphoid Tissue)
Protects mucous membranes of respiratory, digestive, and genitourinary tracts.
Prevents pathogens from entering the body through mucosa.
a. Tonsils
Function: Trap and destroy pathogens entering through mouth and nose.
Structure: Not fully encapsulated; contain crypts to trap bacteria; contain lymphoid follicles.
b. Peyer’s patches
Located in the ileum of small intestine.
Monitor gut bacteria and generate immune response.
Contain follicles similar to lymph nodes but lack capsule.
c. Appendix
Located at junction of small and large intestine.
Contains lymphoid follicles.
Believed to help maintain gut flora and immune defense.
7. Thymus
a. Function
Maturation of T lymphocytes.
Produces thymic hormones (e.g., thymosin) that promote T cell development.
Establishes self-tolerance (prevents autoimmunity).
b. Structure
Two lobes enclosed in connective tissue capsule.
Divided into lobules with cortex and medulla.
Cortex: Dense with immature thymocytes.
Medulla: Contains fewer cells and Hassall’s corpuscles (concentric epithelial cell clusters).
No lymphoid follicles; does not directly fight antigens.
Most active and largest in children; involutes with age (replaced by fat).
Important Additional Details
Cisterna chyli: Dilated sac in abdomen that collects lymph from lower body; starts thoracic duct.
Lymphedema: Swelling due to impaired lymph drainage.
Elephantiasis: Severe lymphedema caused by parasitic filarial worms.
High Endothelial Venules (HEVs): Specialized venules in lymph nodes allowing lymphocytes to enter from blood.
Germinal centers: Sites within follicles where activated B cells proliferate and mutate to increase antibody affinity.
Reticular fibers: Fine collagen fibers providing structural support to lymphoid organs.