Fig 1.

The principle of the two-step method of complement activation. Step 1 (upper part) terminates with the formation of the alternative C5 convertase (C3bBbC3b) or classical C5 convertase (C4b2aC3b), respectively. Step 2 (bottom part) begins after removal of excessive serum, C5 blockers and complement inhibitors by washing. Guinea pig serum diluted in ethylenediamine tetraacetic acid (EDTA) buffer is added as a source of membrane attack complex (MAC) components. These conditions allow initiation of MAC formation only by convertases formed in step 1 and disable de-novo convertase formation.