The complement system plays a vital role in the human immune response. When activated, the numerous membrane-bound and serum proteins act in a complicated cascade which can result in the lysis of host cells, bacteria, or viruses and/or promote an inflammatory response. Complement-like molecules have been isolated in a variety of organisms, including sea urchins, suggesting that the complement system has ancient (600-700 million-year-old) origins. (Smith, et al. 1996) It should therefore be no surprise that some organisms have developed mechanisms to evade this effective form of defense. In this section of the website, I will review how the complement system works and the host mechanisms of complement system control. Then I will discuss some specific organisms' complement evasion mechanisms and the implications that these evasive strategies have for vaccine development.