Page 105 - Veterinary Immunology, 10th Edition

C3bBb complex is a protease whose preferred substrate is C3. (It is
VetBooks.ir therefore called the alternative C3 convertase.) FD can act only on

FB after it has bound to C3b but not before. This constraint is called
substrate modulation and regulates several reactions in the

complement pathways. It ensures that the activities of enzymes
such as FD are confined to the correct molecules.
The alternative C3 convertase, C3bBb, cleaves bound C3 to
generate more C3b. C3bBb is, however, very unstable, with a half-

life of only 5 minutes. If a protein called factor P (FP or properdin)
binds to the complex, it forms a stable C3bBbP complex with a half-
life of 30 minutes. Since C3b generates more C3bBbP, the net effect
of all this is that a positive feedback loop is established where

increasing amounts of C3b are produced and irreversibly bound to
the surface of the invading organism. Despite its name, the
alternative complement pathway accounts for 80% to 90% of all
complement activation.

The Lectin Pathway

A second method of activating complement is triggered by the
binding of soluble pattern-recognition molecules (lectins) to
microbial carbohydrates. When these lectins bind to microbes, they

activate proteases that activate complement. Like the alternative
pathway, this is an innate pathway triggered simply by the
presence of bacterial PAMPs (Fig. 4.6).

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