Despite the prevalence of disease-causing microorganisms, the body is not defenseless against these invaders. The body fights infections in three ways: by preventing the organisms from entering the body, by attacking those that do manage to enter, and by inactivating those organisms it cannot kill.
Sometimes, too, the body fights disease by developing defensive symptoms. Fever is an example. During an illness, the body's temperature regulator may respond to the illness by raising the body's temperature. Some researchers believe that this is an effective response because the microorganisms causing the disease may not be able to survive the higher body temperature.
The skin is the first barrier that guards the underlying tissues of the body. Where there are natural openings in the skin, there are also defenses. For example, tear glands in the eyes secrete and bathe the eyes with fluid that contains bacteria-fighting components. The salivary glands in the mouth and the tonsils in the throat help prevent microorganisms from attacking the mouth and throat.
Many openings, as well as internal passages, in the body are lined with mucous membranes. These delicate layers produce mucus, a slippery secretion that moistens and protects by repelling or trapping microorganisms.
Internally, certain body organs fight infection. For instance, the liver and the spleen (a large glandlike organ located in the abdomen) filter out harmful substances from the blood flowing through them. The lining of the stomach produces acids that attack germs in food that has been eaten. The body's lymph system manufactures white blood cells, which attack and kill invading organisms.
Now let's get even more specific in our look at the body's defenses. We'll start by describing the lymph system.