C3

Inflammation

C3 and C5a induce changes that contribute to local vascular permeability and attract phagocytes.

Inflammation

C3 and C5a induce changes that contribute to local vascular permeability and attract phagocytes.

Lysis of foreign cells

Formation of a membrane attack complex, which creates pores in cell membranes, disrupting the integrity of the cell.

p imiim.

p imiim.

Opsonization

C3b binds to microbial cells, functioning as an opsonin.

Opsonization

C3b binds to microbial cells, functioning as an opsonin.

Figure 15.7 Complement System Activation of the complement system leads to inflammation, lysis of foreign cells, and opsonization.The three mechanisms that trigger the cascade include the classical pathway, the lectin pathway, and the alternative pathway. Not all of the steps in these pathways are shown.

mannan, a polymer of mannose typically found on microbial but not mammalian cells. When MBL binds to a surface, it can then interact with the complement component involved in initiating the classical pathway. ■ mannose, p. 384

■ Alternative pathway. The alternative pathway is quite unlike the other pathways in how it is initiated; nearly any cell surface automatically triggers the pathway unless regulatory proteins specifically halt the process. This occurs because one of the complement proteins, C3b, readily binds cell surfaces. Unless regulatory proteins quickly inactivate C3b, a stabilizing protein will bind to it, allowing a subsequent cascade of reactions to occur. Host cell membranes contain molecules that bind those regulatory proteins, facilitating the inactivation of C3b before the alternative pathway is triggered. Those regulatory proteins are generally not associated with microbial surfaces, however, leading to complement activation by the alternative pathway. As we will discuss in chapter 19, some disease-causing bacteria have developed mechanisms to thwart complement activation by this pathway.

The nature of the complement system allows an exceedingly rapid and powerful response. Its activation occurs by a cascade of reactions; once a specific protein becomes activated, it functions as an enzyme, cleaving and therefore activating millions of molecules of the next protein in the cascade. In turn, each of those molecules activates multiple molecules of the next

15.5 Sensor Systems

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