#Id: 7481
                                                        #Unit 4. Cell Communication and Cell Signaling
                                                        IgE 
Present only at very low levels in blood or extracellular fluid but is bound avidly by receptors on mast cells.Antigen binding to this cell-associated IgE triggers mast cells to release powerful chemical mediators that induce reactions such as coughing, sneezing, and vomiting.