Interaction of T and B lymphocytes. The left panel shows interaction of a CD4⁺ Th2 cell with a B lymphocyte. The release of cytokines IL-4 and IL-13 and interaction of CD40L and CD40 result in IgE synthesis and the sensitization of mast cells, which can then be triggered by allergen to activate an acute allergic response. The right panel shows the complex effects of corticosteroids, which increase the expression of CD40L in T and B cells, thereby increasing IgE formation and potentially acute allergic responses. On the other hand, corticosteroids also decrease expression of CD40L and the synthesis of IL-4 and IL-5, thus counteracting these effects.