Page 40 - Veterinary Immunology, 10th Edition

If sometime later a second dose of toxin is injected into the same
VetBooks.ir horse, it is recognized by this much larger population of memory B

cells. As a result, the lag period lasts for no more than 2 or 3 days.
The amount of antibody in serum then rises rapidly to a high level

before declining slowly. Antibodies may be detected for many
months or years after this injection. A third dose of the antigen
given to the same animal results in an immune response
characterized by an even shorter lag period and a still higher and

more prolonged antibody response. As will be described later in
this book, the antibodies produced after repeated injections are
better able to bind and neutralize the toxin than those produced
early in the immune response. This progressive improvement of

adaptive immune responses to infectious agents by repeated
injections of antigen effectively generates memory cells and forms
the basis of vaccination.
The response of an animal to a second dose of antigen is very

different from the first in that it occurs much more quickly,
antibodies reach much higher levels, and it lasts for much longer.
This secondary B cell response is specific in that it can be provoked
only by a second dose of the same antigen. A secondary response

may be provoked many months or years after the first injection of
antigen, although its size tends to decline as time passes. A
secondary response can also be induced even though the response
of the animal to the first injection of antigen was so weak as to be

undetectable. These features of the secondary response indicate that
memory B cells possess the ability to “remember” previous
exposure to an antigen. For this reason, the secondary immune
response is sometimes called an anamnestic response (anamnesko is

Greek for “remembering”).

Cell-Mediated Immunity

If a piece of living tissue such as a kidney or a piece of skin is
surgically removed from one animal and grafted onto another of
the same species, it only survives for a few days before being

rejected and destroyed by the recipient. This process of graft
rejection is significant because it demonstrates a mechanism
whereby foreign cells, differing only slightly from an animal's own

normal cells, are rapidly recognized and destroyed. Even cells with

40