The Immune System
The immune system uses a complex and remarkable communications network to defend the body against infection. Inside the body, millions and millions of cells are organized into sets and subsets. These cells pass information back and forth like clouds of bees swarming around a hive. The result is a sensitive system of checks and balances that produce a prompt, appropriate, and effective immune response.
The topic of this paragraph is “the immune system.” But “the immune system” is not the main idea. The main idea is what the writer is saying about the immune system. Here, the main idea is expressed in the first sentence: The immune system uses a complex and remarkable communications network to defend the body against infection.
Topic/Subject: What the passage is about.
Main idea: The overall fact, feeling, or thought a writer wants to convey about his or her subject.
Much of the writing you will see in textbooks will follow a very basic pattern of general idea + specific support. That is, the writer will state the main idea he or she wants to convey about the topic and then provide support for that idea, usually in the form of specific facts and details.
In the immune system paragraph, for example, the first sentence states the main idea—a general claim about the subject. The rest of the sentences offer specific facts and details to “prove” that the immune system is a complex and effective communication network.
DISTINGUISHING MAIN IDEAS FROM SUPPORTING IDEAS
If you’re not sure whether something is a main idea or a supporting idea, ask yourself the following question: Is the sentence making a general statement, or is it providing specific information? In the paragraph below, for example, most of the sentences (except one) are too specific to be the main idea of the paragraph. Only one sentence—the first—is general enough to serve as an umbrella or net for the whole paragraph.
Snakes
Many people are afraid of snakes, but most snakes aren’t as dangerous as people think they are. There are more than 2,500 different species of snakes around the world. Only a small percentage of those species are poisonous, and only a few species have venom strong enough to kill a human being. Furthermore, snakes bite only 1,000–2,000 people in the United States each year, and only ten of those bites (that’s less than 1%) result in death. Statistically, many other animals are far more dangerous than snakes. In fact, in this country, more people die from dog bites each year than from snakes.
