The present invention relates to variant antibodies and methods of generating said antibodies with a reduced level of binding to process impurities. In particular, the invention describes variant IgG4 antibodies which have been modified in the heavy chain constant region at any one or a combination of amino acids in the region between Kabat residues 203 and 256, wherein the variant IgG4 antibody has a reduced level of binding to host cell protein (HCP), compared to an unmodified IgG4 antibody. The invention also relates to compositions comprising said variant IgG4 antibodies.