The invention provides recombinant protein (e.g., a recombinant antibody or soluble receptor) having (i) a binding domain capable of specific binding to an epitope (for example an antibody variable domains, a receptor, a growth factor, a cytokine, or a fragment of any of the foregoing which is capable of specifically binding the desired epitope) and (ii) an effector domain comprising a constant domain region which is derived from immunoglobulin of a first species which is a companion mammal, e.g. dog, cat, or horse, having engineered substitutions at one or more positions and having an altered interaction with one or more FcRs or other ligands, and optionally enhanced effector function, relative to the parent constant domain region.
