[Activation of the alternative pathway of human complement (author's transl)]

The C3 amplification convertase of human complement, C3b, Bb, is assembled on biological surfaces by the interaction of the alternative pathway proteins B, D and P with cell-bound C3b. Convertase formation is modulated by the action of the regulatory proteins H and I. On activating surfaces of the alternative pathway, C3b, Bb is relatively resistant to regulation by H. In contrast, non-activating surfaces exhibit surface characteristics such as high content in membrane-associated sialic acid or heparin-related material which increase the interaction of H with cell-bound C3b. Thus, finely tuned molecular interactions allow the alternative pathway to function as a major recognition and effector system for natural defence.