Immunosuppression in AIDS might be due to the immune response rather than to the pathogenicity of the virus. The basis of the immunosuppression could be molecular mimicries involving viral gp-110, CD4 molecules, antibodies, and CD4-acceptor sites. Whether an individual develops auto-immunosuppressive responses or mounts a harmless defence against (or coexists with) the virus follows the general rules of lymphocyte repertoire selection. MHC and V region genes and other polymorphic loci, together with the previous state of the immune system, particularly at early developmental periods, are factors that influence the response. Vaccination against gp 110-HIV might thus protect against infection but at the same time cause auto-immunosuppression and disease.