Three-dimensional structure of a fluorescein-Fab complex crystallized in 2-methyl-2,4-pentanediol.

The crystal structure of Fab 4-4-20 in complex with the epitope was determined. The intact 4-4-20 antibody was derived from the same idiotypic family as 9-40 (see complex of 9-40 Fab with fluorescein [PDB: 1T66]). These two antibodies differ by few somatic mutations in the germline genes which leads to a 1000-fold reduction in affinity for fluorescein of 9-40 relative to 4-4-20.