Abstract: The present assays provide ex vivo methods for determining a significant and potentially life threatening response of mast cells and/or basophils of a patient to one or more antigens, where the response is mediated by IgE and the Fc&egr;RI protein binding and serotonin, histamine or other signaling or mediator of IgE-allergen binding. Cells which express IgE and Fc&egr;RI are labeled by uptake of detectable 5-hydroxytryptamine (serotonin) or other signaling compound, and the release of the intracellular pool of detectable compound is triggered by the addition of the allergen to which sensitivity is observed. Significant release in response to a particular compound or composition indicates that there is potentially danger to the patient from whom the cells were prepared if that patient comes in contact with the allergen. The assay can also be manipulated to detect the presence of a potentially dangerous allergen in a composition.