1. Skin: This is the body's first line of defense. It's a tough, continuous layer that acts as a physical barrier against bacteria, viruses, fungi, and parasites. The skin also has a slightly acidic pH and secretes antimicrobial substances that further deter invaders.
2. Mucous Membranes: These line the respiratory, digestive, and urinary tracts. They secrete mucus, a sticky substance that traps pathogens and prevents them from entering deeper tissues. The cilia, tiny hair-like structures present in some mucous membranes, can also help sweep away trapped invaders.
3. Cilia: These tiny, hair-like projections line the respiratory tract and help to sweep away mucus and trapped pathogens, preventing them from entering the lungs.
4. Tears: These contain lysozyme, an enzyme that breaks down bacterial cell walls. They also help to wash away foreign objects from the eyes.
5. Saliva: This also contains lysozyme, as well as other antimicrobial substances that help to kill bacteria in the mouth.
6. Stomach Acid: The highly acidic environment of the stomach kills most ingested bacteria.
7. Normal Flora: The human body harbors a diverse population of microbes called normal flora, which compete with harmful invaders for resources and space, making it harder for pathogens to establish themselves.
8. Body Fluids: Secretions like urine and vaginal discharge help to flush out bacteria and prevent them from colonizing.
These physical barriers work together to form the first line of defense against invading organisms. If these barriers are breached, the body's immune system kicks in to fight off the infection.
