You’re usually unaware of all the microbes roaming the world because you can’t see them and because your innate immunity keeps most of them from bothering you. Innate immunity is the built-in immunity that your body has. Like the walls of a fortress, your innate defenses can repel all attackers (meaning they’re not specifically targeted for one particular pathogen).

Your innate defenses have several ways of fending off the potential pathogens you encounter:

  • Physical barriers: Your skin and mucous membranes are the barriers that physically block access to your tissues and organs. Both the skin and mucous membranes are epithelia, tissues composed of multiple cell layers that are packed tightly together to prevent microbes from sneaking in. Your skin is very dry and strong, which is an additional barrier against infection. Your mucous membrane cells produce sticky mucus that traps microbes.

  • Chemical barriers: The physical barriers of your skin and mucous membranes get an extra boost of protection from their chemistry. For example, the low pH of your stomach acid prevents microbial growth and destroys most microbes that enter your body in food. In addition, your mucus and other fluids in your body contain a variety of defensive proteins that help prevent infection:

    • Lysozyme is an enzyme that breaks down one of the chemicals found in bacterial cell walls. It’s one of the most common molecular defenders in your body, and it exists in your tears, saliva, mucus, perspiration, and tissue fluids. Basically, when bacteria land on you or in you, they encounter lysozyme.

    • Transferrin in your blood binds iron so microbes don’t have enough iron for their growth.

    • Complement proteins in your blood and tissue fluids bind to microbes and target them for destruction.

    • Interferons are released by cells infected with viruses. They travel to cells all around the infected cell and warn them about the virus. Cells that receive a warning from interferon produce proteins to help protect themselves against viral attack.

  • Dendritic cells: These cells use special receptors, called Toll-like receptors, to recognize foreign molecules that make up microbial cells. When dendritic cells detect alien intruders, they alert the cells of your adaptive immunityso your body can fight off the invasion. Dendritic cells break down bacteria and viruses and then present fragments of their molecules to white blood cells, called helper T cells, that coordinate your adaptive immune response. Scientists call the molecules that can activate your immune system antigens.

  • Phagocytes: Phagocytes (literally "eating cells") like macrophages and neutrophils are white blood cells that seek and destroy microbes that have successfully entered your body. They actually wrap around invading microbes and eat them alive. Like dendritic cells, phagocytes activate helper T cells by showing them molecules from the destroyed microbes.

  • Inflammation: When microbes do manage to invade, the microbes and your own damaged cells trigger a cascade of events that leads to inflammation, a defensive response to cellular damage characterized by redness, pain, heat, and swelling. Inflammation fights infection by destroying microbes, confining the infection to one location, and repairing damaged tissue. Molecules such as histamine that are released during inflammation lead to vasodilation and increased blood vessel permeability.

    • Vasodilation causes blood vessels to widen, allowing more blood to flow to the affected area to deliver clotting elements and white blood cells.

    • Increased blood vessel permeability means the blood vessel walls loosen up, allowing white blood cells and materials to leave the blood and enter the tissues.

  • Filters: The mucus in your nose and throat and the hairs in your nose act as filters that trap microbes and prevent them from getting deeper into your body. In the respiratory tract, a blanket of cilia called the ciliary escalator moves mucus upward in the throat to a location where you can cough it out or swallow it, protecting your lower respiratory tract from infection.

For each of the following questions, name the component of your innate defenses that’s the best match for the given description.

  1. This cell engulfs and destroys pathogens.

  2. This layer is one of your primary barriers to infection and protects you by trapping microbes in a sticky material.

  3. This protein breaks down molecules found in bacterial cell walls.

  4. This protein helps cells prepare themselves to fight off a viral attack.

  5. This response helps limit the spread of infection in the body.

  6. This protein binds iron in the body so that bacteria can’t use it for their own growth.

  7. The low pH of this fluid protects you from microbes trapped in your food.

  8. This process widens blood vessels, allowing more blood to flow to a site of infection.

  9. Having these in your nose helps prevent pathogens from getting into your respiratory system.

The following are the answers to the practice questions.

  1. The answer is phagocyte (or neutrophil or macrophage).

  2. The answer is mucous membrane.

  3. The answer is lysozyme.

  4. The answer is interferon.

  5. The answer is inflammation.

  6. The answer is transferrin.

  7. The answer is stomach acid.

  8. The answer is vasodilation.

  9. The answer is hairs (or mucus).