The Complement System: A Decisive System in Body Defence. This article will discuss the complement system's role in the body's defense mechanisms, including its site of origin and mechanism of action. Keywords: cellular immunity| humoral| Mannose-binding lectin (MBL)| Histamine|Body defense system|WBC| Classical Pathway| Lectin Pathway| Properdin or Alternative   P athway Introduction The complement system is crucial in the body's defense against invading pathogens and tumor cells. The complement system is a system of plasma enzymes. The liver synthesizes enzymes of the complement system. It comprises over 30 enzymes circulating in the blood and is responsible for cell killing by humoral and cellular immunity. These enzymes are inactive and become active when stimulated by an antigen-antibody complex or other pathways. When in active form, they work in a sequence of cascade reactions to remove pathogens, kill pathogens, initiate and promote inflammation, an...