Either histamine or ethylenediamine inactivates a factor of guinea pig complement which is essential to immune hemolysis. Complement inactivated by these agents contains the four known hemolytic components. This factor (called C′6) is heat stable and present in the end piece of complement. These characteristics and the similarity of inactivating agents for C′6 and C′4 indicate a close relationship between these fractions. However, titrations of individual sera from seven mammalian species suggest that C′6 is an entity rather than a part of C′4. The ratio of C′6 to C′4 showed no quantitative relationship among the species.
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