Histamine release from human leucocytes. A serum factor necessary for the induction of histamine release and desensitization by protein A.

The ability of human leucocytes to release histamine on protein A treatment is lost when the cells are washed repeatedly. It is, however, possible to restore the sensitivity to protein A treatment by incubating the leucocytes in serum. Treatment of the cells with purified IgG does not restore the activity. The material responsible for the resensitization is eluted both in the second and the third protein peak when serum is chromatographed on Sephadex G-200, indicating the possible existence of several active factors. Material with low immunoglobulin content, but with retained capacity to resensitize leucocytes to release histamine on protein A treatment, was obtained by repeated chromatography of peak III material on the Sephadex G-200 column. Furthermore, material from the second and third peaks from Sephadex G-200 deprived of their IgG by passage through a protein A Sepharose or a DEAE-cellulose column had the same capacity to resensitize the leucocytes as unseparated material. When serum was separated by Pevikon block electrophoresis, most of the activity was detected in the alpha and beta regions but only little in the gamma region. The serum fractionations indicate that neither IgG nor the other immunoglobulins are the factor(s) responsible for resensitizing the leucocytes to release histamine on protein A treatment. Beside being necessary for protein A-induced histamine release, the factor (or factors) is also essential for protein A-induced desensitization of human leucocytes.