Three-dimensional structure of d6 and d7 Ig-like domains of CD22 in complex with m971 Fab.A, crystal structure of CD22d6–d7 in complex with m971 Fab. D6 (in yellow) and d7 (in wheat) adopt a C2-type Ig-domain fold. The N-acetylglucosamine at N634 in d7 is shown as blue sticks. The internal disulfide linkage in each Ig domain (between C529 and C517 in d6; between C616 and C659 in d7) is represented as sticks. Residues 574 to 580 in d6 were not modeled because of high flexibility of this loop. m971 recognizes an epitope on d7 comprising of β-strand C, and loops C–E and E–F. The heavy chain (dark cyan) and light chain (cyan) of m971 are represented as cartoons. B, topology diagram of d6 and d7 C2-type Ig-like domains. The β-sheets are formed by strands ABE and C'CFG in d6 and by strands A'ABE and CFGG' in d7. Fab, fragment antigen-binding; Ig, immunoglobulin.