Antigens possess immunologically active sites called
antigenic determinants
, or
epitopes
. These are smaller, dis-
crete components of the antigen that have a unique molecular
shape, which can be recognized by and bound to a specific
Ig receptor found on the surface of the lymphocyte or by an
antigen-binding site of a secreted antibody (Fig. 13.4). It is
not unusual for a single antigen to possess several antigenic
determinants and, therefore, be capable of stimulating sev-
eral different T and B lymphocytes. For example, different
proteins that comprise the influenza virus may function as
unique antigens (A, B, C, H, and N antigens), each of which
contains several antigenic determinants. Hundreds of anti-
genic determinants are found on structures such as the bacte-
rial cell wall.
Low molecular weight molecules (<10,000 Da) may
contain antigenic determinants but alone are usually unable
to stimulate an immune response. These molecules are known
as
haptens
. When they are complexed with an immunogenic
carrier (usually a protein), they function as antigens. Many
haptens exist in nature and frequently create problems for
humans. Urushiol is a toxin found in the oils on poison ivy that
Epitopes
(antigenic
determinants)
Antibody
A
Antibody
C
Antibody
B
Antigen-
binding
sites
Antigen
FIGURE 13.4
•
Multiple epitopes on a complex antigen being recog-
nized by their respective (
A
,
B
,
C
) antibodies.
290
UNIT IV
Infection, Inflammation, and Immunity
Understanding
The Complement System
(Continued)
Late-Step Membrane Attack
In the late-step responses, C3b binds to other complement
proteins to form an enzyme that cleaves C5, generating C5a
and C5b fragments. C5a stimulates the influx of neutrophils
and the vascular phase of acute inflammation. The C5b frag-
ment, which remains attached to the microbe, initiates the
formation of a complex of complement proteins C6, C7, C8,
and C9 into a membrane attack complex protein, or pore,
that allows fluids and ions to enter and cause cell lysis.
C5
C5b
C5b
C6,C7,C8,C9
Lysis of
microbe
Membrane
attack
complex
