The invention relates to novel antibodies and their fragments are disclosed that specifically bind to CD4. Anti- CD4 antibodies are disclosed having amino acid substitutions in the hinge region that prevent intrachain disulfide bond formation resulting in antibody molecules with surprisingly improved bivalent stability. Increased stability facilitates manufacturing and provides increased in vivo stability. Antibodies are also disclosed that have amino acid substitutions that reduce binding to Fc receptor and ADCC activity that also result in a surprising effect on CD4 internalization. In addition, genes for such antibodies are disclosed that have been modified such that they have increased expression over their unmodified counterparts.
