https://doi.org/10.1351/goldbook.13612
Relatively heat-labile, normal serum protein (a euglobulin) that, in the presence of complement component C3 and magnesium ions, is involved in the alternative pathway of complement activation, and acts nonspecifically against Gram-negative bacteria and viruses and may contribute to the lysis of erythrocytes.
Note: It migrates as a β3-globulin and, although not an antibody, may act in conjunction with complement-fixing antibody.