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11.60: Innate Immune System

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    What is your nose good for?

    Your nose does a lot of work for you! Obviously, it helps you breathe and provides your sense of smell. But you might not realize that your nose also helps to fight off disease.

    The Immune System's First Line of Defense

    It is the immune system's job to protect the body. Your body has many ways to protect you from pathogens. Your body’s defenses are like a castle. The outside of a castle was protected by a moat and high walls. Inside the castle, soldiers were ready to fight off any enemies that made it across the moat and over the walls. Like a castle, your body has a series of defenses. Only pathogens that get through all the defenses can harm you.

    The first line of defense includes both physical and chemical barriers that are always ready and prepared to defend the body from infection. Pathogens must make it past this first line of defense to cause harm. If this defense is broken, the second line of defense within your body is activated.

    Your body’s first line of defense is like a castle’s moat and walls. It keeps most pathogens out of your body. This is a non-specific type of defense, in that it tries to keep all pathogens out. The first line of defense includes different types of barriers. Being the "first line", it starts with the skin. The first line also includes tears, mucus, cilia, stomach acid, urine flow, and friendly bacteria.

    Skin and Mucous Membranes

    The skin is a very important barrier to pathogens. The skin is the body’s largest organ. In adults, it covers an area of about 16 to 22 square feet! The skin is also the body’s most important defense against disease. It forms a physical barrier between the body and the outside world. The skin has several layers that stack on top of each other (Figure below). The outer layer is tough and waterproof. It is very difficult for pathogens to get through this layer of skin.

    The skin's many layers helps keep most pathogens out
    Figure \(\PageIndex{1}\): This drawing shows that the skin has many layers. The outer layer is so tough that it keeps out most pathogens.

    The mouth and nose are not lined with skin. Instead, they are lined with mucous membranes. Other organs that are exposed to the outside world, including the lungs and stomach, are also lined with mucous membranes. Mucous membranes are not tough like skin, but they have other defenses.

    One defense of mucous membranes is the mucus they release. Mucus is a sticky, moist substance that covers mucous membranes. Most pathogens get stuck in the mucus before they can do harm to the body. Many mucous membranes also have cilia. Cilia in the lungs are pictured below (Figure below). Cilia are tiny finger-like projections. They move in waves and sweep mucus and trapped pathogens toward body openings. When you clear your throat or blow your nose, you remove mucus and pathogens from your body.

    Cilia lining the lungs help sweep mucus and pathogens out of the lungs
    Figure \(\PageIndex{2}\): This is what the cilia lining the lungs look like when they are magnified. Their movements constantly sweep mucus and pathogens out of the lungs. Do they remind you of brushes?


    Most body fluids that you release from your body contain chemicals that kill pathogens. For example, mucus, sweat, tears, and saliva contain enzymes called lysozymes that kill pathogens. These enzymes can break down the cell walls of bacteria to kill them.

    The stomach also releases 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 can grow in it.

    Helpful Bacteria

    You are not aware of them, but your skin is covered by millions (or more!) of bacteria. Millions more live inside your body. Most of these bacteria help defend your body from pathogens. How do they do it? They compete with harmful bacteria for food and space. This prevents the harmful bacteria from multiplying and making you sick.


    • Your body’s first line of defense includes the skin and other barriers that keep pathogens out of your body.
    • Most body fluids that you release from your body contain chemicals that kill pathogens.

    Explore More

    Use the resource below to answer the questions that follow.

    1. How do external barriers help our immune system?
    2. Where is mucus used as a barrier?
    3. How do some bacteria aid our immune system?


    1. How does your skin protect you from pathogens?
    2. What is mucus? What are mucous membranes?
    3. How is mucus helpful?
    4. How are lysozymes helpful?
    5. How do helpful bacteria defend your body?

    This page titled 11.60: Innate Immune System is shared under a CC BY-NC license and was authored, remixed, and/or curated by CK-12 Foundation via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.