Complement component C3d enhances the adaptive immune response to foreign antigens by simultaneously binding to the antigen and to complement receptor type 2 on B lymphocytes. Interaction with C3d requires the first two of 15 or 16 short consensus repeats (SCR) of CR2. Now Szakonyi et al. have determined the crystal structure of the SCR1 and SCR2 domains of human CR2 in complex with C3d at a resolution of 2.0 angstroms. The interface is between C3d and SCR2 and involves mostly main-chain interactions. The SCR domains form a V-shape with side-chain packing. This structure is likely to facilitate targeted drug design.