Introduction - Your Immune System
A Defense against Hostile Invaders

by From the Dana Sourcebook of Immunology

January, 2006

It’s a jungle out there: every day, as you go to school, participate in sports, or just lounge around on the sofa watching television, you are surrounded by thousands of predators, just waiting to pounce. But in this case, the predators are not lions or tigers. They are germs—tiny organisms so small you can see them only with a microscope (which is why they are known as microbes). Some burrow into the soil around your house; others live in the water you drink, the air you breathe, and even the food you eat. Still others cling to doorknobs and phones, settle into your pillows, and hitch a ride on your clothes or skin as you go about your business. Fortunately, your body has a vast army of defenders to guard against invasions by predatory germs. This army is your immune system, and it includes a variety of soldiers, a sophisticated communications system, and highly targeted weapons. It wages a continuous battle against the opposing army of bacteria, viruses, molds, parasites, and other microscopic organisms that can infect your body and cause disease or illness if left unchecked. While most microbes are harmless, and many are even quite helpful, some, if they are not stopped, can make you sick to your stomach, give you a miserable cold, or cause more serious problems—such as pneumonia, food poisoning, and even some types of cancer.

Rabies, seen here under a microscope, is an often fatal viral disease that affects the nervous system. The rabies virus multiplies in the brain, damaging it. Rabies is spread by the bite of an infected animal.  Dr. Fred Murphy 

Older than Humans

Microbes are among the oldest living things on Earth, having existed long before humans. Geologists have found fossil records of bacteria dating back 3.5 billion years. What we now know as polio was depicted in Egyptian stone engravings from 1300 B.C.; the Greek physician Hippocrates described malaria in the fourth century B.C. The famous Ice Man, the remarkably well-preserved Ice Age human discovered in northern Italy in 1991, was found to have the eggs of a type of parasitic roundworm in his intestines.

Scientists first linked microbes to infectious disease in the 1800s, and by the late 1900s it had become clear that microbes were also involved in a number of chronic diseases and conditions. Since microbes are virtually everywhere in our environment, they have many ways to infect us:

  • through airborne particles, such as when someone coughs or sneezes (tuberculosis is an
    example of a disease that spreads this way);
  • via person-to-person contact such as kissing (mononucleosis, colds, flu) or sexual intercourse (herpes, gonorrhea, HIV);
  • through contact with another person or a surface harboring germs (colds; rotavirus, which causes diarrhea);
  • via contact with sick animals (rabies);
  • by the bite of an insect (malaria, Lyme disease);
  • in contaminated food or water (E. coli, hepatitis A).

Ticks are insects commonly found in wooded areas. Some species carry the bacterium Borrelia burgdorferi, which causes Lyme disease. Like mosquitoes, they feed on the blood of other animals, but unlike mosquitoes, they hook into the animal’s skin for extended periods of time. CDC

Modern medical science has increasingly implicated microbes in coronary artery disease, diabetes, autism, multiple sclerosis, chronic lung disease, and certain types of cancer. In fact, microbial infections account for more deaths worldwide than any other single cause, and the cost to treat them exceeds $120 billion a year in the United States alone.

Human Defense System

Your body’s first line of defense against any hostile invader is something you probably take for granted: your skin, the body’s largest organ. Among other health-related duties, the skin protects against biological predators in several ways. Skin has three layers, providing a formidable physical barrier to bugs. Sweat, oils, and other skin secretions help neutralize and wash away invaders. And our skin is populated by harmless bacteria that consume nutrients that would otherwise feed enemy invaders.

 Benjamin Reese

But the barrier that skin provides isn’t foolproof: the eyes, nose, and mouth all provide openings where invaders can sneak in. For this reason, your body has a second set of biological barriers, located in the mucous membranes that line these openings. Every time you blink your eyes, for example, your eyelids wash away microbes much the way windshield wipers sweep away debris. Inhale something that your body knows doesn’t belong, like pollen, and you’ll sneeze the invader out. Saliva and tears both contain the enzyme lysozyme, which destroys bacteria. Any harmful bacteria that somehow manage to sneak down your throat plunge into a deadly acid bath in your stomach.

Unfortunately, wily bugs are sometimes able to breach these multiple barriers. Or germs might sneak in through a temporary open door, such as a cut in the skin. That’s when the battle really begins. 

A Two-Part System

Although the type of soldiers the immune system deploys and the protective strategy it uses depend on the type of invader, your immune system defense generally consists of two basic components, the innate immune system and the adaptive immune system.

You are born with the components of the innate immune system already in place and ready to defend you. These inborn defenders are white blood cells known as phagocytes, which respond quickly whenever they detect a biological bad guy. Phagocytes are fast, but not particularly discriminating: they attack and destroy—literally gobble up—any type of invading germ. Sometimes this is enough to prevent you from getting sick, but more often the phagocytes require additional help. That’s when the second component of the immune system—the adaptive immune system—is activated. The adaptive immune system consists of white blood cells known as lymphocytes. These cells are armed with highly specific traits that enable them to destroy particular types of enemy cells. What’s more, lymphocytes never forget a foe or how to destroy it, which is why adaptive immune system protection is sometimes referred to as acquired or learned immunity.

Although these two components of your immune system are quite distinct, they are enormously effective because they coordinate their response when faced with a threat. Whether you think of the two components of your immune system as first responders and reserves, or as generalists and specialists, the most important point is to think of them as a team. 

Cows, Beer, and Beyond

Gradually, through a series of discoveries over hundreds of years, scientists have learned about the way the immune system works and how to safely harness its protective power. One of the most significant advances in immunology—the creation of the first vaccine, in 1796—resulted from the effort to protect people against smallpox, a disfiguring viral scourge throughout history (and one that has recently risen again as a bioterrorism threat). Over the centuries, people around the world tried to protect themselves against smallpox by exposing themselves to the actual disease (a method known technically as inoculation). An early Chinese emperor, for example, inhaled dried smallpox scabs in an effort to inoculate himself. George Washington inoculated his troops against smallpox during the Revolutionary War by introducing the virus into the skin, then keeping troops in isolation until they were no longer contagious. But these early inoculation efforts, which involved exposure to the unaltered—and therefore quite virulent—smallpox virus itself, killed some people and made others extremely ill.

The world’s first vaccine, developed for smallpox, relied on a similar strategy but provided a much safer alternative. The road to discovery began in the late 1700s, when Edward Jenner, a country doctor, sought to explain why milkmaids and other farmhands exposed to cowpox, a relatively mild disease, did not come down with smallpox. To test the theory that cowpox exposure somehow bolstered a person’s defenses against smallpox, Jenner took pus from a milkmaid infected with cowpox and injected it into a healthy eight-year-old boy. Jenner then exposed the child to smallpox, but the child never succumbed. After several additional experiments, Jenner proved that it was possible to prevent an infectious disease with a vaccine—a word derived from the Latin word vacca, or cow—derived from a substance that was similar but not identical to the real culprit.

Although Jenner’s discovery represented a significant advance, it took many more years before scientists began to understand how diseases occur and to recognize clues that, in the late nineteenth and early twentieth centuries, would reveal why vaccines confer protection.

 In the late 18th century, when smallpox was a big health crisis, Edward Jenner noticed that milkmaids who had been exposed to the milder disease, cowpox, were not contracting smallpox. This led him to take infected pus from people with cowpox and inject it into healthy children. The children in Jenner’s experiment were protected from smallpox.

National Library of Medicine


In the late 1860s, the French scientist Louis Pasteur solved a puzzle that had long plagued brewers and wine makers: why beer and wine sometimes went sour during fermentation. Pasteur discovered that harmful bacteria fouled the brew—an insight that led to investigations into the role bacteria and other microbes might play in disease. Pasteur and a German physician, Robert Koch, separately conducted experiments which gradually proved that various infections were caused by particular microbes. The idea that germs could cause disease, once scoffed at, gradually gained acceptance.

But the manner in which the body defended itself was not clear until the late 1800s. Elie Metchnikoff, a Russian zoologist, decided to do an experiment with starfish larvae. At the larval stage starfish are translucent, so it is possible to see inside them. Metchnikoff inserted a rose thorn into a starfish larva and then waited to see what would happen. The next day, he noticed that the thorn was surrounded by a puslike substance. Metchnikoff discovered that the pus consisted of white blood cells, which seemed to consume infectious microbes. Metchnikoff called the cells phagocytes, from the Greek words phagos (to eat) and cyte (cell). At about the same time, Emil von Behring, a German bacteriologist, discovered that something in the blood of an immunized animal could protect people against diphtheria. Although the substance itself wasn’t known (Behring called it the antitoxin), this finding showed that the body relied on defenses in the blood as well as in cells. Metchnikoff’s discovery marked the beginning of the science of innate immunity, while Behring’s discovery of antibodies was the first example of adaptive immunity.

The discoveries continued as scientists investigated the microbes that cause disease and the cell- and blood-based defenses that provide protection.

In this book, you’ll learn about the immune system in all its complexity. You’ll learn how the immune system can both protect against and—when it mistakenly turns its firepower at the wrong target—contribute to disease. You’ll read about dramatic discoveries that have propelled the field of immunology forward, and about particular diseases, such as AIDS, which have caused much human tragedy. You’ll also learn about the brain’s impact on the immune response. This book may not make you an expert on immunology—few people are—but it should help you more fully appreciate the vast armies of defenders the body continuously relies on for its health.