Key Molecule in Immune System (IMAGE) University of Pennsylvania School of Medicine Caption This shows the molecular structure of the complement pro-convertase in complex with factor D. A double safety catch mechanism that involves a tiered and meticulously orchestrated interplay of three proteins (C3b, factor B, factor D) ensures that the destructive action of complement is restricted to target cells such as microbial intruders. Credit Federico Forneris & Piet Gros (Utrecht University, The Netherlands); Science Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.