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Two morphologically distinct types of circumoval precipitates (COP) have been observed in human Schistosoma japonicum infections. An elongated segmented COP occurs in chronic human infections. An unsegmented “reaction of recent infection” (RRI) occurs in serum from humans with recently acquired infections and is morphologically similar to the reaction observed in the sera of mice infected with S. japonicum. Sera from infected mice and humans were separated by G-200 chromatography to determine whether the unsegmented “RRI” was due to IgM antibody and the segmented COP reaction due to IgG. There was an elevation of the 19S fraction of sera of mice with 10 and 16 week infections. In addition, the murine 7S fraction was elevated in the 16 week infections. The COP activity was confined to the 7S fraction in the murine sera. Sera from Philippine patients which produced reactions of recent infection (acute sera), segmented COP reactions (chronic sera), and mixed reactions (believed to be from a transition stage between acute and chronic schistosomiasis) were tested. All human sera had elevation of both the 19S and 7S globulin fractions. Reactions of recent infection were produced by both the 19S and 7S fractions of the acute serum. However, COP-reactive antibodies were confined to the 7S fraction of sera from the transition stage and acute stage infections. The results suggest that although IgM antibodies do in certain cases participate in the COP and produce reactions of recent infection, antibody class is not responsible for the different morphology of this reaction.