Characterization of a stimulating factor released by immunosuppression.

To get more information about the mechanism of stimulation of the immune response during immunosuppression under certain conditions, stimulating factor (SF) was collected 48 hours after treatment of NMRI-mice with cyclophosphamide. SF was characterized by different steps of separation using centrifugation, dialysis and salt-fractionation. For demonstration of the SF animals were sensitized with SRBC and given different substrates separately. The stimulation of the antibody-production was demonstrated on the cellular level using the plaque-technique. SF was detected by comparing the counts of plaque-forming cells in the spleen of treated mice compared with controls merely sensitized or given the same substrate of normal animals. In this way it was shown that the SF is a soluble factor in the serum with a molecular weight of more than 20,000, but that it is no immunoglobulin.