Figure A: complement cascade

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.

Alternative Pathway:

• 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