The formation of rabbit antibodies, capable of sensitizing homologous skin, (reagins), was completely inhibited by passive administration of serum containing large quantities of 7S antibody 24 hours before or after antigen injection. No evident effect on reagin formation was noted when passive antibody was administered 8 days after antigen injection although some suppression of agglutinating antibody synthesis was observed. In rabbits not treated with passive antibody the injection of haemocyanin resulted in the formation of reagins reaching maximum serum concentrations 1 and 3 weeks following antigen injection. Both the `early' and `late' reagins persisted for a long time in the skin of injected rabbits, they appeared to have similar molecular size and both were devoid of PCA activity when injected into decomplemented rabbits. There was some indication that the `early' reagins may be more heat-labile than the `late' ones. A secondary reagin response was obtained in several animals which had shown a primary reagin response, but not in rabbits with inhibited primary response. The reagins formed in response to secondary antigen stimulation disappeared rapidly from the circulation, simultaneously with the rise in agglutinating antibody titres.
The possible implications of the findings for the immunological treatment of allergic disorders is discussed.