Bodily Defenses Image Map

The Second Line of Defense: The Immune System
Before the immune system was discovered, how do you suppose scientists knew to even look for such a system?

History provides many examples of disease outbreaks that affected some people but not others. Even in the days before we knew about infection, during the black plague of the Middle Ages, for example, thousands of people died, but many others got sick and did not die. This was explained through superstitious myths about good spirits and bad spirits.

When the age of science dawned, it was only natural to want to investigate why some people did not catch certain diseases. How did scientists go about answering this question?

One clue emerged when scientists saw that boils or pustules would develop in the skin as a response to localized injuries, such as splinters in the skin. It was as if something about the injury triggered the formation of pus.

Ever have zits? The next page shows how zits can tell us about the immune system.

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