Complement is a system of at least 20 separate proteins and cofactors that continuously circulate in the bloodstream. Complement acts to kill bacterial cells that are missed by the neutrophils and the macrophages. There are actually two separate complement pathways. One, the classical pathway, operates in the adaptive or acquired immune response. The classical pathway has an absolute requirement for an Ab-antigen complex as a trigger. The other, the alternative pathway, requires no Ab or antigen to initiate and is operative in innate immunity. Both pathways operate in a tightly regulated cascade fashion. The proteins normally circulate as inactive proenzymes. When the pathways are activated, the product of each step activates the subsequent step.
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