Human Physiology, 14th edition (2016)

The Immune System 515

concentrations in the blood during this primary response reach

a plateau in a few days and decline after a few weeks.

A subsequent exposure of that person to the same antigen

results in a secondary response ( fig. 15.20 ). Compared to the

primary response, antibody production during the secondary

response is much more rapid. Maximum antibody concentra-

tions in the blood are reached in less than two hours and are

maintained for a longer time than in the primary response. This

rapid rise in antibody production is usually sufficient to pre-

vent the person from developing the disease.

Clonal Selection Theory

The immunization procedures of Jenner and Pasteur were

effective because the people who were inoculated produced

a secondary rather than a primary response when exposed

to the virulent pathogens. The type of protection they were

afforded does not depend on accumulations of antibodies in the

blood, since secondary responses occur even after antibodies

When the boy recovered, Jenner inoculated him with what was
considered a deadly amount of smallpox, to which the boy
proved to be immune. (This was fortunate for both the boy—
who was an orphan—and Jenner; Jenner’s fame spread, and as
the boy grew into manhood he proudly gave testimonials on
Jenner’s behalf.) This experiment, performed in 1796, began
the first widespread immunization program.
A similar but more sophisticated demonstration of the
effectiveness of immunizations was performed by Louis Pas-
teur almost a century later. Pasteur isolated the bacteria that
cause anthrax and heated them until their virulence (ability to
cause disease) was greatly reduced (or attenuated ), although
their antigenicity (the nature of their antigens) was not sig-
nificantly changed ( fig. 15.19 ). He then injected these attenu-
ated bacteria into 25 cows, leaving 25 unimmunized. Several
weeks later, before a gathering of scientists, he injected all 50
cows with the completely active anthrax bacteria. All 25 of the
unimmunized cows died—all 25 of the immunized animals
survived.

Active Immunity and the

Clonal Selection Theory

When a person is exposed to a particular pathogen for the first
time, there is a latent period of 5 to 10 days before measur-
able amounts of specific antibodies appear in the blood. This
sluggish primary response may not be sufficient to protect the
person against the disease caused by the pathogen. Antibody

Figure 15.19 Virulence and antigenicity. (1) Active
immunity to a pathogen can be gained by exposure to the
fully virulent form or by inoculation with a pathogen whose
virulence (ability to cause disease) has been attenuated (reduced)
without altering its antigenicity (nature of its antigens). (2) Upon
subsequent exposure to the same pathogen, active immunity
reduces the chances of getting the disease.

Virulent pathogen

Not altered Attenuated

Innoculation

Active immunity

Antigens Virulence

1

Virulent

pathogen

2

Disease

No disease

Figure 15.20 The primary and secondary immune

responses. A comparison of antibody production in the

primary response (upon first exposure to an antigen) to antibody

production in the secondary response (upon subsequent

exposure to the antigen). The more rapid production of

antibodies in the secondary response is believed to be due to the

development of lymphocyte clones produced during the primary

response.

0 1 2345

Plasma antibody concentration

Secondary response

Primary response

Week after exposure to antigen