Cascade
It is initiated by the spontaneous hydrolysis of C3, which is abundant in the plasma in the blood. "Tickover" occurs through the spontaneous cleavage of the thioester bond in C3 to form C3(H2O).
This change in shape allows the binding of plasma protein Factor B, which allows Factor D to cleave Factor B into Ba and Bb.
Bb remains part of the C3(H2O) to form C3(H2O)Bb. This complex is also known as a fluid-phase C3 convertase. This convertase, although only produced in small amounts, can cleave multiple C3 proteins into C3a and C3b.
The alternative pathway C3-convertase consists of the activated B and D factors, forming an unstable compound that can become stable after binding properdin, a serum protein.
After the creation of C3 convertase, the complement system follows the same path regardless of the means of activation (alternative, classical, or lectin). Binding of another C3b-fragment to the C3-convertase of the alternative pathway creates a C5-convertase analoguous to the lectin or classical pathway.
The C5-convertase of the alternative pathway consists of C3bBbC3b also referred to as C3b2Bb (instead of C4b2a3b in the other pathways).
Read more about this topic: Alternative Complement Pathway
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