There are pathogens on practically everything we touch. They're in the air we breathe, the food we eat, and the water we drink. In order to cause an infection, a pathogen must get past the external barriers and enter the body.

The skin that covers the body is one type of external barrier. It keeps out most of the pathogens we come into contact with. The mucous membranes that line our respiratory, digestive, urinary, and genital tracts are also external barriers, even though they are inside the body. They're considered external barriers because they are the first thing a pathogen encounters when we breathe the agent in or swallow it.

Every time we inhale, the air and any undesirable agents that might be in the air, first encounter the mucous membranes of our respiratory tract. We'll see in a moment how the mucous membranes protect us against pathogens

Our skin and mucous membranes act not only as physical barriers to pathogens, they also provide chemical protection. Sweat and oil secreted by the skin give the body’s surface a pH between 3 and 5. This pH range is too acidic for most pathogenic microorganisms to survive.

Saliva and tears help wash away potentially infectious agents from our mouth and eyes. Saliva and tears also contain proteins that help destroy pathogens. One of these proteins is lysozyme, an enzyme that digests the cell walls of many types of bacteria.

Some cells in our mucous membranes secrete mucus. Other cells have hairlike projections called cillia. The movements of the cillia push the mucus along the membranes, and any pathogens trapped in the mucus are then either swallowed or expelled. For example, the mucus in your nose traps any infectious agents you breathe in. The cillia in your nose move the mucus and trapped pathogens out of your body. This is what causes your nose to run and makes you sneeze.

If the mucus is swallowed, it enters the digestive tract. There, the pathogens are exposed to the highly acidic liquids of the stomach, which generally destroys them.

While the skin and mucous membranes provide a strong barrier against invading agents, they aren't impenetrable. They can be damaged, thereby allowing pathogens to enter the body. Once pathogens get inside, the body has to fight and destroy them. Let's look at some nonspecific internal defenses our body's immune system uses to destroy invaders.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education