Complement Diagnostics: Concepts, Indications, and Practical Guidelines

Figure 1

Overview of the complement system. Recognition by the lectin and classical pathways leads to the assembly of the C4b2a convertase, which cleaves C3. This reaction is greatly amplified by the alternative pathway, generating more C3b, and ultimately initiating the terminal sequence. The fluid phase anaphylatoxins, C3a and C5a, together with the cell-bound opsonins iC3b and C3d,g, facilitate phagocytosis. The main inhibitor of each step in the cascade is indicated in boxes: C1INH for initiation, C4BP for the classical pathway, factor H for the alternative pathway, and CD59 for the terminal sequence.