Essentials of Anatomy and Physiology

The Lymphatic System and Immunity 335

Primary and secondary antibody responses

Antibody level

First

exposure

to

antigen

Second

exposure

to

antigen

10 days 20 days some months some years 10 days 20 days

Time after exposure

IgG

IgG

IgM IgM

Figure 14–9. Antibody responses to first and subsequent exposures to a pathogen. See

text for description.

QUESTION:State the two differences in IgG production after a first and a second exposure

to the same antigen.

BOX14–4 VACCINES

the inactivated toxins of these bacteria. Vaccines for

pneumococcal pneumonia and meningitis contain

bacterial capsules. These vaccines cannot cause dis-

ease because the capsules are non-toxic and non-

living; there is nothing that can reproduce.

Influenza and rabies vaccines contain killed viruses.

Measles and the oral polio vaccines contain attenu-

ated (weakened) viruses.

Although attenuated pathogens are usually

strongly antigenic and stimulate a protective

immune response, there is a very small chance that

the pathogen may regain its virulence and cause

the disease. The live-virus oral polio vaccine (still

being used in the quest to eliminate polio through-

out the world) has a risk of 1 in 500,000 of causing

polio. The killed-virus injectable polio vaccine has

no such risk.

The purpose of vaccines is to prevent disease. A vac-
cine contains an antigen that the immune system
will respond to, just as it would to the actual
pathogen. The types of vaccine antigens are a killed
or weakened (attenuated) pathogen, part of a
pathogen such as a bacterial capsule, or an inacti-
vated bacterial toxin called a toxoid.
Because the vaccine itself does not cause disease
(with very rare exceptions), the fact that antibody
production to it is slow is not detrimental to the
person. The vaccine takes the place of the first
exposure to the pathogen and stimulates produc-
tion of antibodies and memory cells. On exposure
to the pathogen itself, the memory cells initiate
rapid production of large amounts of antibody,
enough to prevent disease.
We now have vaccines for many diseases. The
tetanus and diphtheria vaccines contain toxoids,