Complement activation is due to a cascade of proteolytic steps, performed by serine protease domains in some of the components. Three different pathways of activation are distinguished triggered by target-bound antibody (the classical pathway); microbial polysaccharide structures (the lectin pathway); or recognition of other "foreign" surface structures (the alternative pathway) by C3b. All three merge in the pivotal activation of C3 and, subsequently, of C5 by highly specific enzymatic complexes, the so-called C3/C5 convertases. A complement system with three C3 activation pathways and a common lytic pathway is found only in jawed vertebrates.