We are constantly exposed to agents that cause disease, such as bacteria, viruses, and fungi. These agents are known as pathogens. Pathogens are present on many of the objects we encounter every day, like this stack of wood. When we pick up a piece of wood, our skin keeps the bacteria on the wood from entering our body.

But skin isn't impenetrable. When picking up the wood, we could get a splinter. The splinter might be covered with bacteria, and the microorganisms could enter the body with the splinter. For most people, getting a splinter is just a minor annoyance. Our body has ways to destroy the bacteria on the splinter, so we don't get sick.

Our body's immune system has three lines of defense for keeping pathogens, like the bacteria on the splinter, from damaging the body. The first line of defense is external -- these barriers prevent infectious agents from getting into the body. External barriers are nonspecific because they act against all types of pathogens. Examples of the immune system's external barriers are our skin and the mucous membranes that line our respiratory, digestive, urinary, and genital tracts. Skin is tough and hard to penetrate, and most objects can't easily pierce it. However, some things, like a splinter, can penetrate the skin, thereby letting bacteria and viruses enter the body. Once inside, pathogens are exposed to our body's second line of defense.

The immune system's second line of defense is internal — a nonspecific response inside the body. This response is nonspecific because the cells and proteins involved attack all types of infectious agents, without targeting a specific invader. Particular types of white blood cells and a large number of proteins destroy the pathogens. The body has mechanisms for increasing the number of these white blood cells and proteins in the area of the infection.

In this activity, we'll look at the body's first two lines of defense. We'll look at the external barriers to infection and how they prevent pathogens from entering the body. Then we'll look at components of the nonspecific immune response and how they destroy infectious agents that do get inside the body. The third line of defense is specific to the particular pathogen, and we'll discuss specific responses in a separate activity.

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