Turbidimetric method for the assay of antiviral antibodies

A rapid, simple assay method has been developed for antiviral antibodies. The technique has been applied to antisera, immune gamma-globulins, and immunospecifically purified antibody for two strains of influenza virus, Asian 305 and PR8, and to antisera to tobacco mosaic virus. Turbidity changes due to the specific interaction of a virus with its antibody were measured by the increase in optical density in a sensitive wavelength region, e.g., 436 nm. Successful application of the method required that nonspecific effects which give rise to turbidity changes be eliminated. This was accomplished by proper choice of ionic strength (0.3 m) and pH (5.5), and by the addition of normal serum or serum albumin to the virus before contact with the antibody. Sensitivity of the method allowed quantitation of antibody down to the level of 10 mug of antibody protein per ml. The specificity of the reaction causing the turbidity change was established by experiments which showed that precipitation of virus-antibody complexes removed the reactive component in the serum, and by the absence of turbidity changes for nonspecific pairs (virus plus unrelated antisera).