The alternative complement pathway in bovine serum: The isolation of a serum protein with factor B activity

A heat labile serum factor (BSF). required for the lysis of unsensitized human erythrocytes by the alternative pathway of bovine complement, has been isolated from bovine serum. The molecular weight of BSF was about 100,000 daltons. A specific antiserum was raised which was able to detect BSF in either serum or in the purified form as a beta-globulin. The antiserum was also used to demonstrate that, in cobra venom factor (CVF) treated bovine sera, the BSF was cleaved into at least two fragments. On the basis of its functional behaviour in haemolytic tests and as an integral component of a CVF-induced C3-convertase activity, BSF appeared to be the bovine counterpart of human factor B.