Tobacco mosaic virus reacts as a single component in immunological systems. As has been found for other protein systems, increasing amounts of antigen added to a constant volume of immune rabbit serum produces increasing amounts of precipitate which pass through a maximum in the region of antigen excess. In general the maximum amount of antibody precipitates in the region of the equivalence zone. The ratio of antibody to antigen in the equivalence zone is variable but has a mean of about 0.25 indicating that about 60 molecules of antibody combine with each virus particle in the complex. The relationship between R and the amount of antigen precipitated is markedly curvilinear but cannot be expressed by any of the Heidelberger and Kendall equations. This curve seems to be typical of virus-antivirus systems. Electronmicrographs of the reaction product between tobacco mosaic virus and its homologous antibody show a “lattice” effect with each virus particle being separated from others by a definite, constant space, presumably the length of the antibody molecule.
Presented before the Division of Biological Chemistry at the 109th meeting of the American Chemical Society, Atlantic City, New Jersey, April, 1946.