The first scientific experiment to hint at what complement is all about appeared in Lancetin 1894 titled “The Action of Rattlesnake Venom Upon the Bactericidal Power of theBlood Serum.“
Its author, U.S. Army surgeon Charles E. Ewing, attempted to answer the question: Whydoes a corpse killed by a rattlesnake bite decompose so much faster than one laid low by abullet? In controlled trials on rabbits, inoculated or not with venom, Ewing found thatthe venomized “blood at the time of death ... had lost all power of resisting theinvasion and multiplication of ... the bacteria of putrefaction, which are normallypresent in the intestine, develop with astounding rapidity, and even before the animal iscold produce this wonderful rapid decomposition.“
Later scientists determined that venom deactivates the serum-complement factors.
-- David N. Leff