The allergic reaction has two phases. When the body comes into contact with a certain allergen for the very first time, it becomes primed or sensitized to react against the certain substance. Macrophages (in blue) phagocytose the allergen particles, then pass on information about the allergen to the B lymphocytes, which produce IgE antibodies. The antibodies bind to basophils and mast cells. At the next encounter with this allergen, the allergic process is activated -- the allergen binds to the IgE antibodies, triggering the mast cell and basophils to release histamine that orchestrates the allergic reaction (here, skin rash).