nique (4) or the tanned erythrocyte procedure (9), it is suggested that these discrepancies may be attributable to the fact that different substances are being measured by the different indicator systems. Thus, one is drawn to the conclusion that either there are several different properdins, or that properdin is a family of cross-reacting antibodies capable of combining with zymosan. The inability to correlate properdin levels determined by the various procedures might then be due to the fact that antigens such as tanned erythrocytes, T2, T6, and T7 phage are measuring a specific portion of the properdin or normal antibody pool which may vary both in quantity and quality from serum to serum.Before the conclusion is drawn that properdin is normal antibody, however, one distinguishing feature of the properdin system should be stressed. This is the demonstration by Pillemer et al.(10) that C' is required not only for the manifestation of the viricidal, bactericidal, and hemolytic reactions attributed to the properdin system but also for the combination of properdin with zymosan and other polysaccharides reactive in the system. While C' may be essential for b?.ctericidal and hemolytic reactions by immune antibody, it is clear that it is not required for the formation of antigenantibody complexes. Thus, if properdin is to be considered normal antibody, it will be necessary to revise our concepts concerning the nature of normal antibody-antigen reactions in order to incorporate C' as an essential cofactor. While it may be premature to postulate a role for C' in such reactions, the multivalent or cross-linking concept of C' a, presented by Weigle and Maurer (11) might be considered with respect to this problem. It is possible to envisage C' as a stabilizer of readily dissociable immune complexes by virtue of its cross-linking activity. Thus, it might both play a role in the formation of certain low-avidity antibody-antigen complexes and serve as an essential component for the manifestation of such unions where bactericidal or hemolytic reactions are involved.
It is hoped that experiments being under-taken may shed further light on the role of C' in properdin or normal antibody reactions, or both (12). References and Notes I. H. Van Vunakis, J. L. Barlow, L. Levsiet Proc. Natl. Acad. Sci. U.S. 42, 391 (1956).nique (4) or the tanned erythrocyte procedure (9), it is suggested that these discrepancies may be attributable to the fact that different substances are being measured by the different indicator systems. Thus, one is drawn to the conclusion that either there are several different properdins, or that properdin is a family of cross-reacting antibodies capable of combining with zymosan. The inability to correlate properdin levels determined by the various procedures might then be due to the fact that antigens such as tanned erythrocytes, T2, T6, and T7 phage are measuring a specific portion of the properdin or normal antibody pool which may vary both in quantity and quality from serum to serum. Before the conclusion i...