Innate Immunity (Non-Specific)
Innate immunity includes the external physical and chemical barriers provided by the skin and mucous membranes. It also includes various internal defenses such as antimicrobial substances, natural killer cells, phagocytes, inflammation, and fever. Innate immunity is an inborn, non-specific response that doesn’t require previous exposure to the antigen. It is an immediate reaction but has no memory once the antigen is cleared.
First Line of Defense: Skin and Mucous Membranes
This is the first line of defense against pathogens providing both physical and chemical barriers.
Physical Barriers
The Epidermis , the outer epithelial layer of the skin with its closely packed keratinized cells provides a good physical barrier. Periodic shedding of the epidermal cells also helps to remove microbes. Bacteria can RARELY penetrate a health epidermis however if the surface is broken such as cuts, burns, or punctures, pathogens may be able to penetrate and cause disease. The epithelial layer of mucous membranes lining the body’s cavities secrete a fluid called Mucus . This fluid lubricates and moistens the cavity surface. Mucus is very viscous trapping many microbes and foreign substances. The nose contains mucus-coated hairs that trap and filter microbes, dust, and pollutants from inhaled air. In the upper respiratory tract, there are Cilia , which are microscopic hair-like projections on the surface of the epithelial cells. The cilia have waving-like motion which propels inhaled dust and microbes that have become trapped toward the throat. Coughing and sneezing accelerate the movement of mucus and entrapped pathogens out of the body. Swallowing mucus sends pathogens to the stomach where gastric juice destroys them. Other fluids are produced by various organs that also help protect the epithelial surfaces of the skin and mucous membranes. The Lacrimal Apparatus of the eyes manufactures and drains away tears in response to irritants. Blinking spreads tears over the eye washing it. Saliva is produced by the salivary glands and washes microbes from the teeth and from the mucous membrane of the mouth similar to how tears wash the eyes.
The Flow of Urine cleanses the urethra and expels microbes.
Defecation also expel microbes. For example, in response to microbes in the GI tract, the smooth muscle of the GI tract begins to contract vigorously resulting in diarrhea which rapidly expels many of the microbes.
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