The Immune System
Lesson 10, Page 2 of 5

Even though we are exposed to the same "germs," not all of us will get sick. This is because some of us have stronger immune systems than others.

The Body's Natural Defenses

From the point of view of our cells, the world is indeed a dangerous place. Billions of microorganisms can be found everywhere—waiting for an opportunity to invade our bodies. For this reason we are equipped with special defenses against such opportunistic invaders. These defenses include the skin; mucus in such places as the lungs, nasal passages, etc.; acids and enzymes in the digestive tract; and—when all else fails—the immune system.

The Skin

The skin provides a barrier which effectively keeps most would-be invaders out of the body. We all know how a cut in the skin can lead to an infection. This is because the integrity of the skin has been broken allowing the invaders to enter.

Mucus in Lungs, Nasal Passages, etc.

Many microorganisms, or "germs," are trapped by the mucus that lines the openings to the body. Without the mucus, these openings would be easy portals of entry. When all goes well, the organisms are killed by this mucus and ejected from the body by such a mechanism as a cough or a sneeze.

Acids and Enzymes in the Digestive Tract

Organisms that enter through the mouth in our food or drink are usually killed by the acids and enzymes in the stomach and intestines.

The Immune System

Despite the above measures of protection, occasionally an unwelcome visitor gets through the first line of defense and starts to create a problem. This is when our immune system is called into action.

Which of the following is not one of the body's natural defenses against foreign invaders?
The skin
The red blood cells
Mucus in lungs and nasal passages
Acids and enzymes in the digestive tract
The immune system