Leucocyte sensitivity to antigenic release of histamine was determined in allergic children receiving injections of allergen extracts in order to ascertain the frequency of development of complete loss of sensitivity (unresponsiveness). Leucocyte unresponsiveness did not occur in any of eight allergic children given large doses of allergen extracts by an intensive regimen of injections. In only two of twenty-three children who had received injections of allergen extracts by a customary dosage regimen, were the leucocytes found to be unresponsive to antigenic histamine release. These findings confirm our earlier reports.

The incidence of complete loss of leucocyte sensitivity during injection therapy in our studies is lower than reported by others. This is attributed to our employment of a more nearly optimal in vitro system for measurement of antigenic histamine release from leucocytes, thus avoiding the false impression of leucocyte unresponsiveness which can result from use of a less sensitive procedure.

Other findings in the present investigation were: (I) concordant fluctuations in leucocyte sensitivity to two unrelated allergens during injections of one of the allergens, indicating non-specific changes in sensitivity; (2) skin sensitivity and bronchial reactivity were not reduced even with marked decrease in peripheral leucocyte sensitivity; (3) significant decrease in titre of serum complement, as occurs in presence of antigen-antibody complex reactions.