Figure B: complement cascade
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Classification
Classical Pathway:
- under most conditions, initiation of the classical pathway of complement requires the interaction of antigen with C1-fixing antibody
- not all classes of immunoglobulin activate the classical pathway
- IgM & IgG subclasses IgG1, IgG2, & IgG3 bind & activate C1.
- the alternative pathway of complement activation does not have an absolute requirement for antibody & does not use C1, C2, or C4
- it is of particular importance before the appearance of antibody during the course of an infection
Mannan-binding lectin pathway:
- third early pathway of host defense also important before the appearance of antibody during the course of an infection