Immunity in lamprey III. Occurrence of the complement-like activity

The serum of Arctic lampreys (Lampetra japonica) lyses rabbit erythrocytes (E^R), with optimal activity at 4°C. This lytic activity is relatively heat stable, is inactivated after incubation with zymosan (5 mg/ml) for 1 hr at 15°C, and is controlled by Mg⁺⁺ dependent factors present in normal lamprey serum. Hemolysis of sheep erythrocytes (E^S) was obtained with lamprey hemolysin using bullfrog serum as complement (C). The optimal conditions were established for sensitizing E^S with lamprey hemolysin to fix bullfrog C. Under the same conditions hemolysis was not induced with zymosan-treated bullfrog C and lamprey serum. These findings suggest that lamprey serum possesses components that function via the alternate pathway, instead of the classical pathway usually found in more advanced vertebrates.