The Non-Identity of the Antigenic and Antitoxic Groups in Diphtheria Antitoxin Free

Diphtheria antitoxin (horse) saturated with a large excess of toxin can nevertheless combine with a rabbit antiserum against horse serum protein. Conversely, antitoxin precipitated by a large excess of the antiserum and presumably saturated with precipitin is almost unaffected as regards its ability to combine with toxin. In confirmation of Smith and Marrack, there are apparently different combining sites in the antitoxin molecule for toxin and for precipitin.