In the absence of chemical methods affording complete isolation and analysis of antibodies, our knowledge concerning the structure and properties of these substances is at best but approximate.

Although theoretically the injection of a host with a mixture of several proteins may well result in the production of multiple antibodies, the extent to which this actually occurs in a given instance has not been more than partially determined. Cytolysis, cyto-agglutination, precipitation, toxin neutralization and opsonification, we recognize as among the various evidences of antibody activity, but to just what extent each of these activities is due to an independent and distinct antibody is far less defined than the generalizations of certain theories concerning immunity might lead us to conclude.

As bearing upon this problem I wish to report certain experimental results, which suggest the co-existence in the same serum of two distinct antibodies, each producing its own type of reaction.

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