Assay for allergenicity in which an antibody is injected intradermally into a test animal followed at a later time (anywhere from \(\pu{3}\) to \(\pu{48 h}\)) with an intravenous injection of a mixture of a test antigen and a dye (typically Evans blue). Increased vascular permeability at the site of antigen-antibody reaction is visualized by extravasation of the dye.