The present invention relates to antibodies or fragments thereof that specifically bind the extracellular domain of FcγRIIB, particularly human FcγRIIB, and block the Fc binding site of human FcγRIIB. The invention provides methods of treating cancer and/or regulating immune complex-mediated cell activation by administering the antibodies of the invention to enhance an immune response. The invention also provides methods of breaking tolerance to an antigen by administering an antigen-antibody complex and an antibody of the invention.
