The alternative pathway
The alternative pathway is phylogenetically older than the classical but was not generally accepted until 1960s. Again, the control reaction is the activation of C3. In contrast to the classical pathway, however, this pathway bypassess antibody, C1, C4, and C2, and it is bacterial cell walls or endotoxin that activates C3. C3b here is unstable and, if an appropriate receptor is not found, it decays and the molecule becomes inactive. However, if a receptor surface is present, then C3b molecules bind and remain active. Then C3b can use factor D and B of the alternative pathway to form the active enzyme C3bBb; this complex then becomes stabilized in the presence of properdin.
The molecule then can chose between two pathways. It can break down more C2, providing more C3b. Or it becomes stabilized to form the C5 convertase of the alternative pathway.