Editing Epitope (section)

===Cross-activity===
Epitopes are sometimes cross-reactive.  This property is exploited by the immune system in regulation by anti-idiotypic antibodies (originally proposed by Nobel laureate [[Niels Kaj Jerne]]). If an antibody binds to an antigen's epitope, the paratope could become the epitope for another antibody that will then bind to it. If this second antibody is of [[IgM]] class, its binding can upregulate the immune response; if the second antibody is of [[IgG]] class, its binding can downregulate the immune response.{{citation needed|date=September 2012}}