The present invention relates to antigen-binding fragment (Fab) and a Fab-effector fusion protein or polypeptide comprising thereof. The Fab of the present invention specifically binds to serum albumin and thereby has extended in vivo half-life. The Fab of the present invention is characterized by not having cysteine residues that are responsible for the interchain disulfide bond in CH1 domain and C kappa L domain as well. The Fab-effector fusion protein or polypeptide of the present invention can be produced in periplasm of E. coli with high yield, and has increased in vivo half-life. Further, the present invention provides E. coli strain which produces various kinds of Fab-effector fusion proteins or polypeptides, and a pharmaceutical composition comprising the fab-effector fusion proteins or polypeptides.
