Figure 4.

The wheal-and-flare reaction in the skin. (A) Normal basal or a non-reactive state for mast cells in tissue. The needle injects a known allergic antigen or allergen. (B) Antigens provoking allergic reactions drive specific-IgE production by plasma cells, assisted by TH2 cells. Surface bound IgE on mast cells is cross-linked by secondary exposure to allergen, causing release of preformed (e.g., histamine) and lipid-derived (e.g., leukotriene) mediators which drive the vascular events of allergy (vasodilatation and vascular fluid leak) manifesting as a “wheal and flare.” This photograph depicts a typical wheal-and-flare reaction in the skin in response allergen injection.