To measure serum IgG antibodies, the sequence of reagents consisted of serum in a series of eight 2-fold dilutions starting at a 100-fold dilution to generate a broad dose-response curve, mouse anti-human IgG (Fc specific) conjugated with horseradish peroxidase (Accurate Chemical and Scientific, Westbury, NY), and ophenylenediamine substrate (KPL, Gaithersburg, MD).