Physical Barriers:
* Skin: The skin is a tough, waterproof barrier that prevents most pathogens from entering the body.
* Mucous Membranes: These line the respiratory, digestive, and urinary tracts. They secrete mucus, which traps pathogens and prevents them from reaching the cells.
* Cilia: These tiny hair-like structures line the respiratory tract and sweep mucus and trapped pathogens up and out of the body.
* Tears and Saliva: These contain enzymes that break down bacterial cell walls.
Chemical Defenses:
* Stomach Acid: The highly acidic environment of the stomach destroys most pathogens.
* Antimicrobial Peptides: These small proteins produced by various cells can kill bacteria, fungi, and viruses.
* Lysozyme: An enzyme found in tears, saliva, and mucus that breaks down bacterial cell walls.
Other Components:
* Normal Microbiota: The body's natural, harmless bacteria compete with pathogens for resources and space, making it harder for them to establish themselves.
It's important to note that the innate immune system is always active, providing constant protection. While it is the first line of defense, it is not always sufficient to fight off infections. The adaptive immune system, which is the body's second line of defense, is responsible for developing a specific response to pathogens.
