Page 844 - Veterinary Immunology, 10th Edition

In disease caused by toxigenic bacteria such as clostridia or Bacillus
VetBooks.ir anthracis, the immune response must not only eliminate the

invading bacteria but also neutralize their toxins. Destruction of the
bacteria, however, may be difficult if they are embedded in a mass

of necrotic tissue, and toxin neutralization is a priority.
Neutralization occurs when antibody prevents the toxin from
binding to its receptors on a target cell. The neutralization process
therefore involves competition between receptors and antibodies

for the toxin molecule. Once a toxin has bound to its cell receptors,
antibodies are relatively ineffective in reversing this combination.

Immunity to Invasive Bacteria

Protection against invasive bacteria is usually mediated by

antibodies directed against their surface antigens. Efficient
phagocytosis requires that the bacteria be coated with opsonins that
can be recognized by phagocytic cells. These opsonins include

antibodies and C3b in addition to the innate opsonins such as
mannose-binding lectin. Antibodies not only are effective opsonins
in their own right but also increase the binding of C3b by activating
the classical complement pathway. Antibodies directed against
capsular (K) antigens may neutralize the antiphagocytic properties

of bacterial capsules, thus permitting their destruction. In bacteria
lacking capsules, antibodies directed against O antigens act as
opsonins. Protection also results when antibodies are produced

against the E. coli pilus antigens F4 (K88) and F5 (K99). The
antibodies may interfere with the expression of pili. Once the
adherence pili are suppressed, these strains of E. coli cannot bind to
the intestinal wall and thus are no longer pathogenic.
The importance of bacterial capsules in pathogenesis is seen in

anthrax. B. anthracis possesses both a capsule and an exotoxin.
Antitoxic immunity is protective but slow to develop. In addition,
toxin production tends to be prolonged since the organism is

encapsulated and phagocytic cells have difficulty eliminating it. As
a result, death is usually inevitable in unvaccinated animals. The
vaccine commonly employed against animal anthrax contains an
unencapsulated but toxigenic strain of B. anthracis. Given in the
form of spores that can germinate, the unencapsulated bacteria are

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