Evolution of the complement system.

The ancestral form of the alternative pathway of complement activation probably originated as a primitive independent immune system. Subsequent evolution of an adaptive immune response drove the specialization of the classical pathway to connect antibody-mediated nonself recognition to the complement-dependent effector mechanisms. In this article Timothy Farries and John Atkinson consider how the contemporary complexity arose by a succession of credible alterations at the genetic level, and the selective advantages provided at each step.