782
UNIT 4
Maintenance of the Body
21
solution. Like agglutinated bacteria, precipitated antigen mole-
cules are much easier for phagocytes to capture and engulf than
are freely moving antigens.
Complement Fixation and Activation
Complement fixation and
activation is the chief antibody defense used against cellular anti-
gens, such as bacteria or mismatched red blood cells. When several
antibodies bind close together on the same cell, the complement-
binding sites on their stem regions align. Tis triggers complement
fixation into the antigenic cell’s surface, followed by cell lysis.
Additionally, as we described earlier, molecules released dur-
ing complement activation tremendously amplify the inflam-
matory response and promote phagocytosis via opsonization.
Tis sets into motion a positive feedback cycle that enlists more
and more defensive elements.
A quick and dirty way to remember how antibodies work is
to remember they have a PLAN of action—
p
recipitation,
l
ysis
(by complement),
a
gglutination, and
n
eutralization.
Inactivates by
Antigen
Antibody
Fixes and activates
Enhances
Enhances
Leads to
Phagocytosis
Chemotaxis
Histamine
release
Inflammation
Cell lysis
Agglutination
(cell-bound antigens)
Precipitation
(soluble antigens)
Neutralization
(masks dangerous
parts of bacterial
exotoxins; viruses)
Complement
Antigen-antibody
complex
Adaptive defenses
Humoral immunity
Figure 21.15
Mechanisms of antibody action.
Antibodies act against free viruses, red
blood cell antigens, bacterial toxins, intact bacteria, fungi, and parasitic worms.
Neutralization
Neutralization
, the simplest defensive mecha-
nism, occurs when antibodies block specific sites on viruses or
bacterial exotoxins (toxic chemicals secreted by bacteria). As a re-
sult, the virus or exotoxin cannot bind to receptors on tissue cells.
Phagocytes eventually destroy the antigen-antibody complexes.
Agglutination
Because antibodies have more than one
antigen-binding site, they can bind to the same determinant
on more than one antigen at a time. Consequently, antigen-
antibody complexes can be cross-linked into large lattices.
When cell-bound antigens are cross-linked, the process causes
clumping, or
agglutination
, of the foreign cells. IgM, with 10
antigen-binding sites, is an especially potent agglutinating agent
(see ±able 21.4). Recall from Chapter 17 that agglutination oc-
curs when mismatched blood is transfused (the foreign red
blood cells clump) and is the basis of tests used for blood typing.
Precipitation
In
precipitation
, soluble molecules (instead of
cells) are cross-linked into large complexes that settle out of
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