York County Medicine Winter 2020 - 20

YO R K C O M E D S O C . O R G

FEATURE:

Understanding How
Vaccines Work
The Immune System-The Body's
Defense Against Infection
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
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York County Medicine | W I N T E R 2020/2021

vaccines, vaccine preventable 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
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York County Medicine Winter 2020

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