Maintenance of the Body
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—
Fixes and activates
parts of bacterial
Mechanisms of antibody action.
Antibodies act against free viruses, red
blood cell antigens, bacterial toxins, intact bacteria, fungi, and parasitic worms.
, 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.
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
, 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.
, soluble molecules (instead of
cells) are cross-linked into large complexes that settle out of