Immunoglobulin M antibodies with ten combining sites

Immunoglobulin M rabbit antibodies to a hapten are shown to have ten binding sites per molecule. The affinity for the specific hapten is approximately 100 times greater for one-half of the sites than for the other half. All sites are retained in the five 7S subunits produced by reduction and alkylation of the immunoglobulin M. Each of the 7S subunits of the IgM molecule apparently has one strong and one weak site.