The incidence of allergy has increased over the last few decades and it is estimated that 30-40% of the world population suffer from an allergic disease such as eczema, hay fever or asthma. In allergic disorder allergen-specific immunoglobulin E (IgE) together with mast cells are the key players of the development of the allergic reaction. Mast cells are one of the most important effector and immunoregulatory cells in allergy and are activated by IgE cross-linked by specific allergens. Allergen-specific IgE binds to the high affinity IgE receptor (Fc℮RI) expressed on the surface of mast cells leading to cross-linking of IgE-bound Fc℮RI and resulting in the release of pro-inflammatory mediators. Given their important roles in the pathophysiology of allergic disorders we are trying on one hand to gain closer insights into the mechanisms underlying IgE production by B cells with an emphasis on CD23, the low affinity receptor for IgE. CD23 has been generally associated with negative regulation of IgE but is also known to promote IgE-facilitated antigen uptake. This leads to subsequent activation of allergen-specific T cells and the production of allergen-specific IgG in vivo. On the other hand, we are aiming to get a better understanding of the biology of the mast cells especially in the complex interplay of positive and negative signaling leading to their activation and the release of Th2 cytokines. This may skew in allergy patient to a Th2-driven response leading to the overproduction of IgE.