Mucosal-associated lymphoid tissue (MALT) is your body's first line of defense against invaders. It's found in places like your gut and lungs, where germs often try to enter. MALT has special cells that catch bad guys and alert your immune system.

MALT is like a neighborhood watch for your body. It has different parts working together to keep you safe. Some cells sample what's in your gut, while others produce antibodies to fight off threats. It's a complex system that helps you stay healthy.

Structure and Function of Mucosal-Associated Lymphoid Tissue

Structure of mucosal-associated lymphoid tissue

• MALT forms diffuse lymphoid tissue found in mucosal surfaces throughout body
  • Located in respiratory tract (lungs, nasal passages), gastrointestinal tract (stomach, intestines), and urogenital tract (bladder, urethra)
• Components of MALT work together to provide immune defense:
  • Epithelial layer contains specialized M cells that sample antigens from lumen
  • Lamina propria houses diverse immune cells (T cells, B cells, plasma cells) that respond to threats
  • Organized lymphoid structures like Peyer's patches and isolated lymphoid follicles coordinate immune responses
• Cellular composition includes:
  • T lymphocytes (both CD4+ helper and CD8+ cytotoxic)
  • B lymphocytes that can differentiate into antibody-producing plasma cells
  • Dendritic cells that present antigens to activate T cells
  • Macrophages that phagocytose pathogens and present antigens
• MALT produces secretory IgA as primary mucosal antibody
  • IgA dimers secreted across epithelium to provide protection in lumen

Role of lymphoid organs in immunity

• Peyer's patches in small intestine ileum serve as inductive sites
  • Contain germinal centers where B cells proliferate and mature
  • Sample antigens from gut lumen to initiate immune responses
• Tonsils in pharynx act as first line respiratory/digestive defense
  • Palatine, lingual, and pharyngeal tonsils trap pathogens
  • Produce antibodies against inhaled or ingested antigens (dust, pollen, bacteria)
• Appendix at small/large intestine junction maintains gut health
  • Houses beneficial bacteria to repopulate gut after illness
  • Contains high concentration of lymphoid tissue for local immunity

Inductive vs effector sites

• Inductive sites like Peyer's patches initiate immune responses
  • Organized lymphoid structures present antigens to naive lymphocytes
  • Include isolated lymphoid follicles and mesenteric lymph nodes
• Effector sites carry out activated immune responses
  • Diffuse distribution throughout lamina propria and epithelium
  • Activated lymphocytes migrate here to fight pathogens
• Lymphocytes travel between sites via lymphatics and blood
  • Allows coordinated immune response across mucosal surfaces

Importance of microfold cells

• M cells sample and transport antigens across epithelial barrier
  • Located in follicle-associated epithelium overlying lymphoid tissue
  • Lack absorptive microvilli found on other epithelial cells
  • Have folded basolateral membrane to increase surface area
• Antigen sampling occurs through several steps:
  1. Endocytosis of antigens from lumen
  2. Transport across cell in vesicles
  3. Release to antigen-presenting cells beneath
• M cells bridge lumen and immune system
  • Enable immune surveillance of mucosal surfaces
  • Crucial for initiating responses to pathogens
  • Help maintain tolerance to commensal microbes