An immunologically specific method was developed for the detection of reaginic antibodies in sera of allergic individuals. It is based on the determination of histamine release (HR) from rat peritoneal mast cells (RPMC), presensitized with the allergic serum, on challenge with the appropriate allergen. The amount of HR was shown to depend on the concentrations of reagins and allergens. From experiments involving RPMC sensitized with different fractions of allergic serum isolated by chromatography or with appropriate reverse immunosorbent, it was deduced that the bulk of HR was due to antibodies of the IgE class and that about 10 of HR was attributable to antibodies of the IgG type; however, paradoxically, IgG of normal human serum did not have the capacity to sensitize RPMC. Both classes of immunoglobulins present in reaginic serum and capable of sensitizing RPMC were inactivated at 56 °C or with 0.1 M 2-mercaptoethanol. Peritoneal mast cells or a rat, which had been immunized to produce homocytotropic (HCT) antibodies, or RPMC sensitized in vitro with rat HCT antibodies, did not fix human reagins. By contrast, human reagins to two distinct allergens were fixed on the same RPMC preparation by successive sensitization of the cells with the corresponding sera. The cells sensitized with rat HCT antibodies released histamine on challenge with anti-human IgE serum. It is therefore concluded that there exist structural homologies between the groups of human and rat reaginic antibodies which are responsible for fixation on rat RPMC, as well as between the antigenic groups of these two reaginic antibodies capable of crossreacting with anti-human IgE
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