774
UNIT 4
Maintenance of the Body
21
Antigens
(an
9
tĭ-jenz) are substances that can mobilize the
adaptive defenses and provoke an immune response. Tey are
the ultimate targets of all adaptive immune responses. (
Antigen
is a contraction of “
anti
body
gen
erating”—the result of mobiliz-
ing one branch of adaptive immunity.) Most antigens are large,
complex molecules (both natural and synthetic) that are not
normally present in the body. Consequently, as far as our im-
mune system is concerned, they are intruders, or
nonself
.
Complete Antigens and Haptens
Antigens can be
complete
or
incomplete
.
Complete antigens
have two important functional properties:
Immunogenicity
, which is the ability to stimulate specific
lymphocytes to proliferate (multiply).
Reactivity
, which is the ability to react with the activated
lymphocytes and the antibodies released by immunogenic
reactions.
An almost limitless variety of foreign molecules can act
as complete antigens, including virtually all foreign proteins,
many large polysaccharides, and some lipids and nucleic acids.
Of these, proteins are the strongest antigens. Pollen grains and
microorganisms—such as bacteria, fungi, and virus particles—
are all immunogenic because their surfaces bear many different
foreign macromolecules.
As a rule, small molecules—such as peptides, nucleotides, and
many hormones—are not immunogenic. But if they link up with
the body’s own proteins, the adaptive immune system may recog-
nize the
combination
as foreign and mount an attack that is harm-
ful rather than protective. (We describe these reactions, called
hypersensitivities
, later in the chapter.) In such cases, the trouble-
some small molecule is called a
hapten
(hap
9
ten;
haptein
5
grasp)
or
incomplete antigen
. Unless attached to protein carriers, hap-
tens have reactivity but not immunogenicity. Besides certain drugs
(particularly penicillin), chemicals that act as haptens are found in
poison ivy, animal dander, detergents, cosmetics, and a number of
common household and industrial products.
Antigenic Determinants
Te ability of a molecule to act as an antigen depends on both its
size and its complexity. Only certain parts of the antigen, called
antigenic determinants
, are immunogenic. Free antibodies or
lymphocyte receptors bind to these antigenic determinants in
much the same manner that an enzyme binds to a substrate.
Most naturally occurring antigens have a variety of antigenic
determinants on their surfaces, some more potent than oth-
ers in provoking an immune response
(Figure 21.7)
. Different
lymphocytes “recognize” different antigenic determinants, so
a single antigen may mobilize several lymphocyte populations
and stimulate formation of many kinds of antibodies.
Large proteins have hundreds of chemically different antigenic
determinants, which accounts for their high immunogenicity
and reactivity. However, large simple molecules such as plastics,
which have many identical, regularly repeating units (and so are
not chemically complex), have little or no immunogenicity. Such
substances are used to make artificial implants because the sub-
stances are not seen as foreign and rejected by the body.
Self-Antigens: MHC Proteins
A huge variety of protein molecules dot the external surfaces of
all our cells. Assuming your immune system has been properly
“programmed,” your
self-antigens
are not foreign or antigenic
to you, but they are strongly antigenic to other individuals. (Tis
is the basis of transfusion reactions and gra± rejection.)
Among the cell surface proteins that identify a cell as
self
is
a group of glycoproteins called
MHC proteins
. Genes of the
major histocompatibility complex (MHC)
code for these pro-
teins. Because millions of combinations of these genes are possi-
ble, it is unlikely that any two people except identical twins have
the same MHC proteins. Each MHC protein has a deep groove
that holds a peptide, either a self-antigen or a foreign antigen.
As we will describe shortly, ² lymphocytes can only bind anti-
gens that are presented (displayed to them) on MHC proteins.
Check Your Understanding
6.
Name three key characteristics of adaptive immunity.
7.
What is the difference between a complete antigen and a
hapten?
8.
What marks a cell as “self” as opposed to “nonself”?
For answers, see Appendix H.
Cells of the Adaptive Immune
System: An Overview
Compare and contrast the origin, maturation process, and
general function of B and T lymphocytes.
Define immunocompetence and self-tolerance, and
describe their development in B and T lymphocytes.
Name several antigen-presenting cells and describe their
roles in adaptive defenses.
Antigenic determinants
Antigen-
binding
sites
Antibody A
Antibody B
Antibody C
Antigen
Figure 21.7
Most antigens have several different antigenic
determinants.
Antibodies (and related receptors on lymphocytes) bind
to small areas on the antigen surface called antigenic determinants.
In this example, three different types of antibodies react with different
antigenic determinants on the same antigen molecule.
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