The extent to which antigenic determinants of hemoglobin are quantitatively or qualitatively modified on complex formation with haptoglobinhas has been examined. From the data on Ouchterlony double immunodiffusion, it has been apparent that the antigenic determinants of hemoglobin are not modified nor masked by complex formation with human or rabbit haptoglobin, and that new antigenic determinants of hemoglobin do not appear in the molecule. The precipitability of hemoglobin-human haptoglobin complex with anti-hemoglobin serum was much greater than that of the hemogglobin with the same antiserum. Hemoglobin-rabbit haptoglobin complex is also precipitated with anti-hemoglobin serum more readily than free hemoglobin, but the difference is not so marked as for the human moiety. With respect to evoking antibody against hemoglobin in rabbits, the antigenicity of hemoglobin associated with rabbit haptoglobin in vitro is stronger than that of free hemoglobin. With this in mind, in order to clarify one of the explanations of increased antigenicity of hemoglobin associated with haptoglobin, the susceptibility of hemoglobin-haptoglobin complex to cathepsins and other proteases has been compared with that of free hemoglobin. Based on the results, it has been concluded that the resistance to these protease of hemoglobin associated with haptoglobin may be one possible explanation for the stonger antigenicity of hemoglobin-haptoglobin complex compared with free hemoglobin.
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