The complement system is part of the innate immune system that helps defend the body against pathogens. It was discovered in 1894 and consists of proteins that flow freely in the blood and can quickly reach sites of infection. When activated, complement proteins trigger inflammation, attract immune cells, coat pathogens for destruction, and can even directly kill pathogens. There are three pathways of complement activation - the classical, lectin, and alternative pathways. Complement activation leads to the formation of the membrane attack complex that causes lysis of pathogen cell membranes.