Cartoon describing the possible pathways of inhibition or of priming/activation by quercetin (Q) in the activation models explored in the study. Putative sites sensitive to quercetin are indicated close to the target proteins PI3Ks or calmodulin: for PKC, Q is included in a dashed area, indicating no effect of the flavonoid on the kinase. Membrane markers are indicated by squares (CD63) or triangles (CD203c). Arrows indicate links between different activation pathways while dashed arrows indicate precursors or metabolites incoming to the interior of the cell from membrane and/or from the outside. For further explanation and comments see the text. BTK: Bruton's tyrosine kinase; DAG: diacylglycerol; GAB-2: Grb-associated binding protein-2, an adaptor protein serving as principal activator of PI3K; GRK: G-coupled receptor kinase; IP3: inositol-1,4,5-triphosphate; PLC: phospholypase C; PtdIns 3,4-P2: phosphatidylinositol 3,4-biphosphate; PtdIns 4,5-P2: phosphatidylinositol 4,5-biphosphate; PtdIns 3,4,5-P3: phosphatidylinositol 3,4,5-triphosphate; Syk, Lyn: signaling tyrosine kinases linked to IgE-high affinity receptor; other abbreviations as in the text.