An anaphylaxis inhibitory factor present in mouse serum

We have previously described that normal mouse serum inhibits the PCA reaction mediated by IgE. The present study attempts to characterize this PCA inhibitory factor from the biologically active fraction of the serum. The physicochemical properties of this glycoprotein are the following: it is inactivated at 55 degrees C; it has a molecular weight between 182 kD and 240 kD, determined by gel filtration; it shows affinity to concanavalin A and lentil lectin but not to peanut agglutin; it demonstrates affinity to IgE and, apparently, its carbohydrate moiety is not required for its biological activity. Two bands corresponding to 64.5 kD and 48. 1 kD, which are likely to constitute the biologically active molecule, are observed by SDS-PAGE. These properties are different from those found in factors with IgE affinity involved in IgE synthesis.