What is a Vaccine? - Definition, Function & Examples

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  • 0:01 What Are Vaccines?
  • 0:38 How the Immune System Works
  • 2:18 How Vaccines Work
  • 3:24 Vaccines and the Flu
  • 3:54 Types of Vaccines
  • 4:35 Lesson Summary
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Lesson Transcript
Instructor: Wendy McDougal

Wendy has taught high school Biology and has a master's degree in education.

Without vaccines, many deadly diseases would have devastated a large portion of the human population. In this lesson, we will learn about how vaccines work and why they have great importance in our world.

What Are Vaccines?

Do you remember as a child heading to the doctor for a checkup and asking the all-important question: 'Do I have to get a shot?' You dreaded the quick, sharp pain of the needle and wondered why you were subjected to this torture. You knew it was a vaccine, but what exactly is that?

A vaccine is an inactivated form of bacteria or virus that is injected into the body to simulate an actual infection. Because the injected microorganisms are 'dead,' they don't cause a person to become sick. Instead, vaccines stimulate an immune response by the body that will fight off that type of illness.

How the Immune System Works

To better understand vaccines, we need to know more about how our body's immune system works. There are special cells in our bloodstream called white blood cells . They have the very important job of fighting off foreign invaders such as viruses and bacteria. These invaders are known as antigens. White blood cells are like the armed forces of our body. They are constantly on the lookout for antigens that have entered our body, compromising our health.

We also have a group of defensive proteins circulating in our blood that are known as antibodies. They float around in non-active form until triggered by an immune response, such as the detection of an antigen. When this happens, billions of additional antibodies are produced that will fight off that particular antigen. This enormous army of antibodies now joins in the attack with the white blood cells, and the germs don't stand a chance.

For example, imagine that an influenza virus has entered your body and has begun replicating. The white blood cells patrolling your bloodstream have spotted these antigens. They gather their troops, produce a few billion antibodies geared to fight this specific virus, and launch a massive attack.

It will take some time for the body to completely fight off these germs, and that's why you have symptoms of the illness for a short time. However, if you have a healthy and strong immune system, you will be as good as new in a few days.

The great news is that now your body has developed a very strong army of antibodies for that particular virus. They remain on the lookout for that same antigen to invade. The next time it enters your body, it will be overtaken by the immune response so fast that you won't even feel any symptoms.

How Vaccines Work

And so how do vaccines work to help us stay healthy? Let's use the measles as an example. As mentioned earlier, within a vaccine are inactivated versions of the virus. Basically, the shells of the virus are present, but their ability to replicate has been taken away, so there is no danger of getting sick from having the vaccine injected into your body.

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