PRRS is the most costly disease U.S. swine producers confront and little progress has been made in recent years to improve the situation. Prevention and control of PRRS has been difficult, in part because our knowledge of the immunity against PRRSV is limited. Recently, we produced a monoclonal anti-idiotypic antibody specific for a monoclonal antibody to the PRRS virus envelope glycoprotein GP5. This antibody functionally and/or structurally mimicked GP5 antigen of PRRS virus, inhibited the binding of anti-GP5 antibodies to PRRS virus, directly bound MARC-145 cells and porcine alveolar macrophages and reacted with a soluble protein prepared from MARC-145 cells and PAM. It blocked PRRS virus infection of MARC-145 cell and PAM and after affinity purified remained the immunological function by recognition with the monoclonal anti-idiotypic antibody. The information on the future characterization of the cellular receptor protein will lead to the development of an immune therapeutic agent, the anti-idiotype-based receptor-blocker and further elucidation of the mechanism of the receptor-virus interaction.