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Chapter 13 : The Classical Pathway of Complement
Complement evolved in parallel with coagulation as part of the primordial explosion of life in the Cambrian era. Remnants of this connection still exist in vertebrate animals as well as invertebrates. A classic example is the horseshoe crab, Limulus polyphemus, still sought after by scientists today to test substances for traces of bacterial lipopolysaccharide (endotoxin). The system used by the crabs had a molecule that recognized bacteria or other invaders that got into the hemolymph of the crab. An enzyme triggered by the first molecule induced local coagulation that served to trap the microbes so that they could be destroyed by phagocytes in the animal's circulation.