Tobacco mosaic virus (TMV) combines with its homologous antibody to much the same extent irrespective of whether or not salt is present, but without salt the complex not only fails to precipitate, but the virus particles do not aggregate. TMV—antibody precipitate formed in the presence of salt, like that formed between human serum albumin (HSA) and its homologous antibody, dissolves when suspended in distilled water to form stable and transparent solutions, although the precipitate may not disaggregate completely.
To dissolve HSA—antibody complex in distilled water, the pH of the water must be raised to about 7.0. At pH near 6.0, HSA—antibody complex precipitates even in the absence of salt, but the precipitate dissolves immediately when the pH is raised to 7.0.
All these facts are incompatible with the theory of precipitation based on the `lattice hypothesis', and argue strongly in favour of the theory that antigen—antibody complexes are hydrophobic and, as such, flocculate when sufficiently discharged either by salt or by suitably adjusting the pH of the medium.