Principle of the basophil activation test by flow cytometry (triple staining). Basophils are identified on the basis of CD45 expression (fluorescence 3 / Phyco-Cyanine 5) and the presence of IgE or CRTH2/DP2 on their surface (fluorescence 1 / Fluorescein isothiocyanate). Resting basophils do not express CD63 (anchored in the basophilic granule) and weakly express CD203c. The cross-linking of two FcεRI (induced by an allergen or anti-IgE antibodies) provokes the histamine release (and as a consequence the CD63 expression) and the upregulation of CD203c. The rise in CD63 or CD203c expression (measured by fluorescence 2 / Phycoerythrin) before and after allergen challenge reflects thus the basophil activation / degranulation in response to an allergen.