26.2 External barriers prevent pathogens from entering your body.

Just as a high-security building may have walls and barbed wire to keep out potential intruders, your body has ways to prevent pathogens from entering your body, tissues, and cells. The integumentary system is made up of skin, mucous membranes (such as those lining the respiratory system), sweat glands, oil glands, hair, and nails, all of which protect you from stress or injuries caused by external forces such as friction, pressure, sunlight, dehydration, and, of course, pathogens (FIGURE 26-5).

Have you ever sat down on a public toilet seat only to wonder if you were going to contract some horrible disease from it (FIGURE 26-6)? The good news is that most human pathogens cannot survive outside the body for very long, and a cold, smooth toilet seat is not an ideal place for them to survive. Furthermore, pathogens need some way to get into your body, and the skin on your buttocks is generally a strong barrier to entry for any pathogens on the toilet seat. Many public bathrooms offer toilet seat covers that can act like a second skin, further reducing the already very low risk.

Skin is not just a passively protective wall, however. It’s more like a wall with armed guards. In addition to forming a nearly impenetrable barrier, skin cells actively secrete chemicals that inhibit bacterial growth (FIGURE 26-7). Sweat secreted onto the skin’s surface, for example, contains lysozyme, an enzyme that kills bacteria by damaging their cell walls. The protection provided by skin becomes really clear in burn victims: lacking skin in certain areas, they are susceptible to infection by numerous pathogenic bacteria and fungi. The external and internal surfaces of the body, such as eyes, ears, and the linings of the digestive and respiratory tracts, also protect us from environmental stresses and forces. These body areas, too, are shielded by the integumentary system (and, as we’ll see, by the cells of the non-specific defense system).

Consider, for example, how the digestive tract is protected. Imagine that minutes after you push a germ-laden shopping cart, you put your finger in your mouth to bite your nail. Are you guaranteed to get ill? No, because mucus (a major component of the saliva in your mouth) also contains the antibacterial chemical lysozyme. And even if lysozyme doesn’t destroy potential invaders, the pathogens will quickly reach the stomach, a highly acidic environment flooded with digestive enzymes that can destroy a variety of pathogens.

In the respiratory tract, sticky mucus traps pathogens before they can reach the lungs. The epithelial cells that line the respiratory tract have hair-like extensions, called cilia, that continually move mucus-entrapped pathogens up and out of the lungs. As the mucus moves up into your throat, you continually swallow the germ-containing mucus and send it to your stomach, where the germs are destroyed. Mucus that’s not swallowed continues up further into your throat until you feel the urge to “clear your throat” or cough.

The eyes produce tears that physically wash away microorganisms, and that also contain lysozyme and antiviral and antibacterial chemicals. Ear wax serves a protective function, too. Secretions produced by numerous glands inside the ear canal, combined with dead skin cells, make up this sticky substance that is effective at trapping microorganisms. The slight acidity of ear wax and the lysozyme the wax contains also serve to inhibit the growth of some microorganisms that find their way into the ear canal.