A Misleading “Reaction of Identity” in Agar Diffusion Precipitin Studies of Tetanus Antitoxin

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Abstract
Summary: The displacement of zone growth by apparently nonspecific antibody—a phenomenon previously reported—is shown to be illusory. Comparable effects may be produced by specific antibody when used in sub-threshold concentrations. At the same time, evidence is offered to show that the presence of heterospecific antibody in a particular region of agar does not significantly change the path of zone growth through that region. The quantitative analysis of several antibodies in a polyvalent serum is therefore not complicated by possible additive concentration effects between physically similar molecules. Internal evidence as to the relative antigen-antibody concentrations used in the present experiments suggests that the phenomenon was produced by qualitative, rather than quantitative relationships in this particular case. It is suggested that antibody formed in horses inoculated with toxoid for prophylactic purposes may have differed in specificity from antibody formed in horses used for tetanus antitoxin production. If so, the former was directed toward a portion of the toxin molecule which was altered or masked during the detoxification process, while the latter was specific for a determinant group which remained unchanged during the exposure of the antigen to formaldehyde. Attention is called to the fact that, on logical grounds, the reaction of identity cannot be used in ascertaining the specificity of an unknown antibody unless it can be shown that antibodies from two different sources necessarily choose the same determinant group for their site of attachment to the antigen molecule. Since antibody from the scarlatinal antiserum is shown to compete with the antibody in tetanal antiserum for the same antigen, this principle may be said to have presumptive experimental support.