Recombinant antibodies and methods for their production in which surface residues are altered to cysteine residues for attachment of effector or receptor molecules

An altered antibody molecule wherein an amino acid residue in a surface pocket on the molecule has been changed to a cysteine residue to introduce a thiol group in said surface pocket, such that said cysteine residue can only be accessed by small molecules of about 0.13 to 0.5 nm in diameter and such that said cysteine residue is only available for bonding to an effector or reporter molecule and not to a cysteine residue on the same or other antibody chains.