Physical Barriers:
* Skin: The skin is a tough, continuous barrier that prevents most microbes from entering the body.
* Mucous Membranes: These line the respiratory, digestive, and urinary tracts, trapping pathogens and secreting mucus to help remove them.
* Cilia: These tiny hair-like structures found in the respiratory tract sweep mucus and trapped pathogens upward, where they can be coughed or sneezed out.
Chemical Barriers:
* Sebum: Oil secreted by the skin creates an acidic environment that inhibits bacterial growth.
* Sweat: Contains lysozyme, an enzyme that breaks down bacterial cell walls.
* Stomach Acid: Extremely acidic environment that kills most ingested pathogens.
* Saliva: Contains lysozyme and other antimicrobial agents.
Cellular Defenses:
* Phagocytes: White blood cells like neutrophils and macrophages engulf and destroy pathogens.
* Natural Killer (NK) Cells: These lymphocytes destroy infected cells and cancer cells without needing to recognize specific antigens.
* Inflammation: A localized response characterized by redness, swelling, heat, and pain. It brings immune cells to the area and helps to isolate and destroy pathogens.
Other Nonspecific Defenses:
* Fever: A rise in body temperature that can inhibit pathogen growth and enhance immune responses.
* Interferons: Proteins produced by virus-infected cells that signal other cells to produce antiviral defenses.
* Complement System: A group of proteins in the blood that can kill bacteria, opsonize pathogens (make them easier to be engulfed by phagocytes), and promote inflammation.
It's important to remember that these nonspecific defenses are not always sufficient to protect against all pathogens. They can be overwhelmed by large numbers of pathogens or by highly virulent pathogens. The adaptive immune system, which provides specific defenses, comes into play when the innate immune system fails to contain the infection.
