Explain how vaccines work. Make sure you differentiate between old and newer vaccines in your response

To understand how vaccines work, it helps to first look at how

the body fights illness. When germs, such as bacteria or viruses,

invade the body, they attack and multiply. This invasion, called

an infection, is what causes illness. The immune system uses

several tools to fight infection. Blood contains red blood cells,

for carrying oxygen to tissues and organs, and white or immune

cells, for fighting infection. These white cells consist primarily of

macrophages, B-lymphocytes and T-lymphocytes:

• Macrophages are white blood cells that swallow up and digest

germs, plus dead or dying cells. The macrophages leave behind

parts of the invading germs called antigens. The body identifies

antigens as dangerous and stimulates antibodies to attack them.

• B-lymphocytes are defensive white blood cells. They produce

antibodies that attack the antigens left behind by the macrophages.

• T-lymphocytes are another type of defensive white blood cell.

They attack cells in the body that have already been infected.

The first time the body encounters a germ, it can take several days

to make and use all the germ-fighting tools needed to get over the

infection. After the infection, the immune system remembers what

it learned about how to protect the body against that disease.

The body keeps a few T-lymphocytes, called memory cells, that go

into action quickly if the body encounters the same germ again.

When the familiar antigens are detected, B-lymphocytes produce

antibodies to attack them.

How Vaccines Work

Vaccines prevent diseases that can be

dangerous, or even deadly. Vaccines greatly

reduce the risk of infection by working with

the body’s natural defenses to safely develop

immunity to disease. This fact sheet explains

how the body fights infection and how vaccines

work to protect people by producing immunity.

Last updated July 2018

CVG15-CHD-158 8/08/2018 H

For more information on vaccines, vaccinepreventable diseases, and vaccine safety:

www.cdc.gov/vaccines/conversations

antibodies. Sometimes, after getting a vaccine, the imitation infection

can cause minor symptoms, such as fever. Such minor symptoms are

normal and should be expected as the body builds immunity.

Vaccines help develop immunity by imitating an infection. This

type of infection, however, almost never causes illness, but it

does cause the immune system to produce T-lymphocytes and

Once the imitation infection goes away, the body is left with a supply of

“memory” T-lymphocytes, as well as B-lymphocytes that will remember

how to fight that disease in the future. However, it typically takes a

few weeks for the body to produce T-lymphocytes and B-lymphocytes

after vaccination. Therefore, it is possible that a person infected with a

disease just before or just after vaccination could develop symptoms and

get a disease, because the vaccine has not had enough time to provide

protection.

Types of Vaccines

Scientists take many approaches to developing vaccines. These

approaches are based on information about the infections (caused

by viruses or bacteria) the vaccine will prevent, such as how germs

infect cells and how the immune system responds to it. Practical

considerations, such as regions of the world where the vaccine would be

used, are also important because the strain of a virus and environmental

conditions, such as temperature and risk of exposure, may be different

across the globe. The vaccine delivery options available may also differ

geographically. Today there are five main types of vaccines that infants

and young children commonly receive in the U.S.:

• Live, attenuated vaccines fight viruses and bacteria. These vaccines

contain a version of the living virus or bacteria that has been weakened

so that it does not cause serious disease in people with healthy immune

systems. Because live, attenuated vaccines are the closest thing to a

natural infection, they are good teachers for the immune system.

Examples of live, attenuated vaccines include measles, mumps, and

rubella vaccine (MMR) and varicella (chickenpox) vaccine. Even

though they are very effective, not everyone can receive these vaccines.

Children with weakened immune systems—for example, those who are

undergoing chemotherapy—cannot get live vaccines.

• Toxoid vaccines prevent diseases caused by bacteria that produce toxins

(poisons) in the body. In the process of making these vaccines, the

toxins are weakened so they cannot cause illness. Weakened toxins are

called toxoids. When the immune system receives a vaccine containing

a toxoid, it learns how to fight off the natural toxin. The DTaP vaccine

contains diphtheria and tetanus toxoids.

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