Activation of the complement sequence through the alternate pathway with zymosan or the low molecular weight cobra venom factor (CoVF) generates products which block the lysis of hemolytic cellular intermediates of the classical complement sequence. The euglobulin fraction of the eluate of zymosan and products in CoVF-treated serum bind to EAC14limoxy2 and EAC14limoxy23 to prevent effective hemolytic interaction with C3 and C5, respectively. Analyses of fractions from gel filtration of both the zymosan eluate and CoVF-treated serum revealed blocking in the IgM region of EAC14limoxy2, EAC14limoxy23, and EACl-7 while materials eluting in the trough between the IgM and IgG peaks blocked only EAC14limoxy2 and EAC14limoxy23. Radiolabeled C5 in CoVF-treated serum was shown to be present in the material eluting in both regions representing a shift from the usual position of C5. Only the highest molecular weight material destroyed C9. It is proposed that the blocking of the cellular intermediates is due to complexes ranging in composition from C5 and C6 to C5, C6, C7 and C8, which are hemolytically inactive while maintaining certain functions, such as binding to the appropriate cellular intermediate or to one or more later components in the complement reaction sequence.

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Supported by Grants AI-07722 and RR-05669 from the National Institutes of Health, and a grant from the John A. Hartford Foundation, Inc.

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