The serum from allergic individuals inhibits the antigenically induced release of histamine from sensitive human leukocytes. This is in contrast to the effect of normal serum, which enhances histamine release. On the basis of this observation, an in vitro method for the study of the reaction between ragweed antigen and the anti-ragweed activity contained in allergic sera has been presented. Over a wide range of antigen concentrations, an allergic serum binds a constant percentage rather than a constant quantity of the added antigen. This outcome is demonstrated to be predictable from the mass action law under the limiting condition of extreme antibody excess. Kinetic and multiplicity studies of the reaction are compatible with this formulation.

Determinations of the antigen-neutralizing capacity of allergic sera provide an index of those immunoglobulins in allergic serum which possess anti-anaphylactic properties. The results of these determinations differ only slightly when performed on the cells of the different allergic donors. In agreement with this observation is the demonstration that the concentration of cell-bound antibody is considerably less than that of serum antibody and does not compete with the latter for antigen.

Antigen-neutralizing activity has been demonstrated in every allergic serum tested, irrespective of previous desensitization therapy. The level of this activity in a group of untreated individuals and its increase as a result of immunization has been presented.

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