What Activates the Complement System?
There are two main mechanisms to activate the complement proteins: the classical pathway, which was discovered first; and the alternative pathway, which is more common. In this sense a pathway is a series of reactions that lead to C3 being cleaved, which in turn leads to the complement system springing into action. The alternative pathway occurs in response to yeast, fungus and bacteria cell walls in combination with some chemicals and C3b, which is always present in blood in small amounts.
