An antigen/antibody specificity exchanger is disclosed. It comprises: A) an amino-acid sequence corresponding to an amino-acid sequence of an antibody which specifically binds to a certain antigen, including hapten, B) linked by a link to C) an amino-acid sequence to which a certain antibody binds. Also, a diagnostic reagent comprising an antigen/antibody specificity exchanger according to the invention is disclosed. Said reagent may be e.g. used instead of antisera or monoclonal antibodies in in vitro testing systems, such as immunological tests. Further, a method of treating a disease or disorder caused by a known antigen in an individual in need of an increased number of antigen-specific antibodies is disclosed. In the method a tailor-made antigen/antibody specificity exchanger of the invention is issued. Said method may be e.g. used to redirect a patient's antibodies against poliovirus to fight HIV infection in said patient.