Model for molecular differences when 2 antibodies simultaneously bind to distinct epitopes on a single surface antigen. (A,B) A single IgG monoclonal antibody recognizing an epitope that occurs once on a surface molecule forms antigen dimers, but not multimolecular complexes. (C) The combination of 2 antibodies recognizing 2 distinct epitopes doubles the number of antibodies bound but also leads to multimolecular crosslinking. (D) The combination of intact IgG and F(ab′)2 fragments, each of which recognizes different epitopes, also induces multimolecular crosslinking but does not have an increased number of exposed Fc domains compared with a single IgG monoclonal antibody.