Mast cells (MC) and eosinophils are the key effector cells of allergy (Minai-Fleminger and Levi-Schaffer, Inflamm Res 58:631-638, 2009). In general, allergic reactions have two phases, namely, an early phase and a late phase. MC and eosinophils abundantly coexist in the inflamed tissue in the late and chronic phases and cross talk in a bidirectional manner. This bidirectional interaction between MC and eosinophils is mediated by both physical cell-cell contacts through cell surface receptors such as CD48 receptors CD48, 2B4 , 2B4 and soluble mediators through various specific granular mediators, arachidonic acid metabolites, cytokines cytokines , and chemokines, collectively termed the "Allergic Effector Unit" (AEU) (Elishmereni et al., Allergy 66:376-385, 2011; Minai-Fleminger et al., Cell Tissue Res 341:405-415, 2010). These bidirectional interactions can be studied in vitro in a customized coculture system of MC and eosinophils derived from either mouse or human source.