Solid phase enzyme-linked immunosorbent assay (ELISA) for anti-sheep erythrocyte antibody in mouse serum

The solid phase enzyme-linked immunosorbent assay (ELISA) was developed to measure IgM and IgG anti-SRBC antibody titers in mouse serum. Sheep erythrocytes, which have a surface negative charge, were attached directly to the bottom of an aminoplate well (96-well flat bottom, Sumitomo Bakelite Co.) which is charged positively to provide a solid phase for the ELISA antigen. Serum samples titrated were obtained from mice 5 and 10 days after SRBC immunization. Alkaline phosphatase-labeled goat anti-mouse IgM and/or IgG preparations were used as second antibody. Alkaline phosphatase activity in a well was measured by the Kind and King method [Kind, P. R. N. & King, E. J. (1954). J. clin. Path., 7, 322-326]; the optical density (OD) value of quinone as a final product was measured at 492 nm. (1) A linear relationship was observed between the antiserum concentration and OD value over a wide enough dilution range to assay antibody titers of serum samples; (2) IgM and IgG fractions from antiserum showed almost the same antibody titer as did the reconstituted samples; (3) a good correlation was observed between the serum IgM titer and the number of IgM hemolytic plaque-forming cells (PFCs) in spleen 5 days after the immunization, and was also observed between serum IgG antibody titer and the number of IgG PFCs 5 and 10 days after. Therefore, the ELISA described here requires no fixative for preparation of cell-coated plates, and serum IgM and IgG anti-SRBC antibody titers can be measured without any fractionation technique.