Alternative complement pathway in bovine serum: lysis of human erythrocytes

The hemolysis of unsensitized human erythrocytes by fresh bovine serums was investigated. Lysis occurred in ethylene glycol bis-amino tetraacetate buffers and with serums depleted of Clq. Serums extensively absorbed with packed human erythrocytes at 0 C effectively lysed human erythrocytes, but optimal lytic capacity required target cells "sensitized" with a heat-stable serum factor. Lysis did not occur with serums absorbed with zymosan at 17 C or heat inactivated at 50 C. These results indicate that human erythrocytes can activate the alternative pathway of complement in bovine serums. Lysis can proceed in the apparent absence of antibodies, although their presence may enhance the reaction.