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Alternative Pathway of Complement System
The other pathway of the complement system is the alternative pathway. Unlike the classical pathway, the alternative pathway does not require the presence of antibody-antigen complexes to activate itself. This is because one of the major proteins of the complement system, the C3 protein, can activate itself spontaneously via a hydrolysis process. Under normal conditions (in the absence of pathogenic agents), the spontaneous breakdown of C3 into C3a and C3b is not a problem because our healthy cells contain an inhibitory protein on their membrane that can quickly bind C3b and inactivate it. However, pathogenic agents such as bacterial cells do not have this inhibitory membrane protein and so cannot deactivate it. In the presence of pathogenic agents, the C3b can quickly combine with another protein called Factor B to form a complex called C3b-Bb complex. This complex is a C3 convertase and is the major player of the alternative pathway. It can do one of two things. It either (1) combine with another C3b to form the C3b-Bb-C3b complex, which goes on to activate C5 and form the membrane attack complex or it can (2) activate more C3 molecules into C3a and C3b. The latter step is an amplification step. Once the complement system curbs the pathogenic agent, our body must be able to turn it off. It turns out that there are over ten different types of proteins that are used to deactivate the classical and alternative pathway. For instance, Factor I is a protein that deactivates C3b in the alternative pathway while the C1-inhibitor inhibits the C1 complex from being activated in the classical pathway.
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