Context: Your bodys first line of defense is like a castles moat and walls. It keeps most pathogens out of your body. The first line of defense includes physical, chemical, and biological barriers. The skin is a very important barrier to pathogens. It is the bodys largest organ and the most important defense against disease. It forms a physical barrier between the body and the outside environment. The outer layer of the skin, called the epidermis, consists of dead cells filled with the protein keratin. These cells form a tough, waterproof covering on the body. It is very difficult for pathogens to get through the epidermis. The inside of the mouth and nose are lined with mucous membranes. Other organs that are exposed to substances from the environment are also lined with mucous membranes. These include the respiratory and digestive organs. Mucous membranes arent tough like skin, but they have other ways of keeping out pathogens. One way mucous membranes protect the body is by producing mucus. Mucus is a sticky, moist secretion that covers mucous membranes. The mucus traps pathogens and particles so they cant enter the body. Many mucous membranes are also covered with cilia. These are tiny, hair-like projections. Cilia move in waves and sweep mucus and trapped pathogens toward body openings. You can see this in the diagram in Figure 21.10. When you clear your throat or blow your nose, you remove mucus and pathogens from your body. In addition to mucus, your body releases a variety of fluids, including tears, saliva, and sweat. These fluids contain enzymes called lysozymes. Lysozymes break down the cell walls of bacteria and kill them. Your stomach contains a very strong acid, called hydrochloric acid. This acid kills most pathogens that enter the stomach in food or water. Urine is also acidic, so few pathogens are able to grow in it. Your skin is covered by millions of bacteria. Millions more live inside your body, mainly in your gastrointestinal tract. Most of these bacteria are helpful. For one thing, they help defend your body from pathogens. They do it by competing with harmful bacteria for food and space. They prevent the harmful bacteria from multiplying and making you sick. Did you ever get a splinter in your skin, like the one in Figure 21.11? It doesnt look like a serious injury, but even a tiny break in the skin may let pathogens enter the body. If bacteria enter through the break, for example, they could cause an infection. These bacteria would then face the bodys second line of defense. If bacteria enter the skin through a splinter or other wound, the area may become red, warm, and painful. These are signs of inflammation. Inflammation is one way the body reacts to infections or injuries. It occurs due to chemicals that are released when tissue is damaged. The chemicals cause nearby blood vessels to dilate, increasing blood flow to the area. The chemicals also attract white blood cells to the area. The white blood cells leak out of the blood vessels and into the damaged tissue. You can see an animation of the inflammatory response by watching this video: MEDIA Click image to the left or use the URL below. URL: The white blood cells that go to a site of inflammation and leak into damaged tissue are called phagocytes. They start eating pathogens and dead cells by engulfing and destroying them. This process is called phagocytosis. You can see how it happens in Figure ??. You can see it in action in the animation at this link: [Link] Phagocytes also release chemicals that cause a fever. A fever is a higher-than-normal body temperature. Normal human body temperature is 98.6 F (37 C). Most bacteria and viruses that infect people reproduce quickly at this temperature. When the temperature rises higher, the pathogens cant reproduce as quickly. Therefore, a fever helps to limit the infection. A fever also causes the immune system to make more white blood cells to fight the infection. 
 Question: __organ that is the bodys most important barrier to pathogens
Output: skin