Complement system
Chapter: 2
A group of 20 plasma proteins that exist, under normal conditions, in inactive forms. When the complement "cascade" is initiated, the enzymatic activation of one component triggers activation of the next. There are two means of activation, the "classical" and "alternative" pathways. The former requires antibody, while the latter does not. For example, endotoxin, a product of Gram negative bacteria, will suffice. The alternative pathway is particularly valuable, therefore, in instances where the immune system has not yet had time to produce antibody that will activate the cascade by the classical route.