Proteins 1. Complement is a system of plasma that circulates in the blood in an inactive form. Complement is activated when it becomes attached to the surface of foreign cells (bacteria, fungi, red blood cells). The result of this complement fixation is that some of the complement elements, called the complement proteins, become inserted in the membrane of the foreign cell. This forms a complex which causes lysis of the foreign cell. Some of the chemicals released during complement fixation enhance phagocytosis. This is called opsonization. Others amplify the inflammatory response.