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.
Explanation:
