The formation of the alternative pathway C5 convertase (C3bBbC3b) starts by spontaneous cleavage of C3 protein exposing previously hidden thioester bond. In the presence of pathogen the fragment C3b binds to microbial cell-surface through the newly showed thioester bond. On the other hand, if the infection does not occur, C3b interacts with molecules of water, therefore the protein becomes inactive. However, when C3b undergoes its post-cleveage conformational change, a binding site for a plasma protein called "Factor B" is also exposed. Factor B then binds to C3b and is cleaved by a plasma serine protease "Factor D". The C3bBb complex (= alternative pathway C3 convertase) remains attached to the cell-surface. This complex might interact with another C3b and thus form the alternative pathway C5 convertase.