A hemagglutination test capable of demonstrating antibodies in sera of ragweed-sensitive individuals has been described in detail. This test involves coupling of the antigen to rabbit erythrocytes via stable azo bonds. The antigen-coated cells are then suspended in the serum which is suspected to contain the homologous antibodies. The test has been shown to be specific and highly sensitive. Precipitating antibodies to ragweed pollen extract produced in experimental animals can be demonstrated in dilutions as high as 105 to 107, while antibodies to ragweed in allergic sera are detected only in dilutions of the order of 103. Positive results were obtained with all sera from treated or non-treated allergic individuals containing skin-sensitizing and/or blocking antibodies. Absorption of these sera with antigen-coated erythrocytes yielded a supernatant devoid of either skin-sensitizing, blocking, or hemagglutinating capacity. It is concluded that the hemagglutinating factor has the properties of a divalent antibody.

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