Residual infectivity detected after the interaction of Venezuelan equine encephalomyelitis virus with specific antiserum was caused mainly by the formation of infective virus + antibody complexes (sensitized virus) that could be neutralized by serum containing antigamma globulin (IgG). The quantities of virus sensitized by antiserum to Venezuelan equine encephalomyelitis and neutralized by anti-IgG serum depended on the antibody concentrations of these sera. In contrast to the marked temperature and time-dependence of Venezuelan equine encephalomyelitis virus neutralization by antiserum, neutralization of sensitized virus by anti-IgG serum was more rapid, being almost complete within 1 min. at 35°, and less sensitive to temperature. Virus sensitization preceded neutralization and indicated that infective virus + antibody complexes were formed before virus neutralization began. The neutralization of sensitized virus by anti-IgG serum was generally species specific. Differences in the ability of anti-IgG, anti-IgA, and anti-IgM sera to neutralize sensitized virus indicated that the reaction was also influenced by the class specificity of the anti-immunoglobulin.

Sensitized virus was partially neutralized by goat antiserum to monovalent Fab fragments of human IgG and, to a lesser degree, by the Fc fragment. Sensitized virus was neutralized by an mixture of these fragments to almost the same degree as by goat antiserum to intact human IgG. The Fc fragment may, therefore, by involved in virus neutralization.


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