1. Physical Barriers:
* Skin: The outer layer of skin, the epidermis, is a tough, continuous barrier that is difficult for pathogens to penetrate. It also contains antimicrobial peptides and fatty acids that inhibit microbial growth.
* Mucous Membranes: These line the respiratory, gastrointestinal, and urogenital tracts. They secrete mucus, a sticky substance that traps pathogens.
* Cilia: Tiny hair-like structures found in the respiratory tract that sweep mucus and trapped pathogens upwards, away from the lungs.
* Tears: Contain lysozyme, an enzyme that breaks down bacterial cell walls.
* Saliva: Contains lysozyme, as well as other antimicrobial agents.
* Gastric Acid: The highly acidic environment in the stomach kills most pathogens that are ingested.
* Urine: The flow of urine helps flush out pathogens from the bladder and urethra.
2. Chemical Barriers:
* Antimicrobial Peptides: These are small proteins that kill pathogens by disrupting their cell membranes.
* Lysozyme: An enzyme that breaks down bacterial cell walls.
* Fatty Acids: These are present in the skin and have antimicrobial properties.
* Sweat: Contains lactic acid and other antimicrobial agents.
* Sebum: An oily substance secreted by the skin that inhibits microbial growth.
3. Physiological Barriers:
* Normal Flora: The bacteria that normally reside on the skin and in the gut, mouth, and vagina can compete with pathogens for resources and space, preventing them from establishing themselves.
* Temperature: The core body temperature of humans is generally too high for many pathogens to survive.
* pH: The slightly acidic pH of the skin and vagina inhibits the growth of many pathogens.
These first-line defenses are constantly working to keep pathogens at bay. If they are breached, the body has other defenses, such as the immune system, to fight off the infection.
It's important to note that these defenses can be compromised by factors like cuts and wounds, weakened immune systems, or environmental conditions.
