Normal human serum can enhance the rate and extent of immune histamine release from ragweed-sensitive leukocytes under specified experimental conditions. The quantitative expression of this potentiation is governed largely by the amount of antigen in the reaction system, being most marked when the release process is limited in this respect. The enhancing effect diminishes with higher antigen levels, becoming entirely inapparent with amounts of antigen which suffice for a maximal response.

The potentiating activity of serum is retained after heating at 56°C and treatment with ammonium hydroxide. Serum so treated is shown to be markedly depleted with respect to C′1, C′4, and over-all complement activity. Moreover, this depletion was not offset by a contribution of C′1 from the leukocytes.

Two inhibitors of complement activity, diisopropylfluorophosphate and salicylaldoxime, rendered the leukocytes incapable of a response to antigen. Since the inhibition was manifest even after the cells were washed and resuspended in optimal buffer, it cannot be decided whether these compounds act to derange homeostasis in the leukocyte or to suppress a reaction step specific to the histamine release process.

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