Activation and covalent attachment of complement component C3 to pathogens is the key step in complement-mediated host defense. Additionally, the antigen-bound C3d fragment interacts with complement receptor 2 (CR2; also known as CD21) on B cells and thereby contributes to the initiation of an acquired humoral response. The x-ray crystal structure of human C3d solved at 2.0 angstroms resolution reveals an a-a barrel with the residues responsible for thioester formation and covalent attach ment at one end and an acidic pocket at the other. The structure supports a model whereby the transition of native C3 to its functionally active stale involves the disruption of a complementary domain interface and provides insight into the basis for the interaction between C3d and CR2.