Desensitization in the process of histamine secretion induced by antigen and dextran.

1. Antigen challenge of sensitized rat peritoneal mast cells in the absence of calcium failed to release histamine. The release when calcium was added subsequently declined rapidly, this desensitization being almost complete in 4 min.2. Phosphatidyl serine (10 mug/ml.) reduced the rate of desensitization so that decay of the response to added calcium was not complete after 16 min.3. Exposing normal cells to dextran resulted in a slow rate of desensitization, the response to phosphatidyl serine added with calcium having decayed by only 27% within 10 min.4. Phosphatidyl serine added with dextran prevented desensitization so that the response to subsequently added calcium did not decay even after an interval of 20 min.5. Cells activated by dextran and calcium became rapidly desensitized as shown by decay of the response to added phosphatidyl serine which was almost complete by 5 min.6. Histamine release by the calcium ionophore (A 23187) added to cells at intervals before the addition of calcium did not show significant decay.7. Desensitization of the cells to antigen did not change their response to the ionophore.