Antibody labelling. The affinity purified immunoglobulin (IgG) was conjugated to colloidal gold (40 nm). The optimal concentration of antibody for conjugation with col¬loidal gold was determined by titrating aliquots of diluted IgG with colloidal gold. The purified IgG was diluted to a concentra¬tion of 0.1 mg/ml in sodium phosphate buffer.