Innate Immune Defenses

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Physical/External barriers, Chemical/Internal Defenses, and various leukocytes form the innate immune defenses. Unbroken skin and mucous membranes are key to preventing pathogen entry. Oil, sweat, and body secretions (tears, saliva, vaginal secretions, and wastes such as urine all have antimicrobial defenses. Oil keeps the skin from cracking and the short-chain fatty acids inhibit microbes. Sweat and tears are both salty and contain lysozyme; salt and lysozyme can inhibit microbe growth. Vaginal secretions and urine are acidic, which inhibits many pathogens. Our mucous membranes produce sticky mucus that traps microbes; ear wax works similarly. Internally, inflammation results in leaky blood vessels that flood a wound or infection site with fluids and white blood cells. Epithelial cells and white blood cells produce a huge variety of antimicrobial peptides and fatty acids. The "Complement system includes over 30 small circulating substances that enhance inflammation and phagocytosis, and come together to form Membrane Attack Complex, which punches holes in pathogen cell membranes. Most white blood cells are part of the innate immune response: neutrophils (the most common circulating white blood cell), basophils, and mast cells produce histamine which triggers inflammation, eosinophils are key in tackling parasitic helminths, and phagocytic antigen-presenting cells display antigens from pathogens and thereby activate and link with the adaptive immune response, which involves T and B cells.