An allergic response comprises two main parts:
The first time an allergy-prone person encounters an innocuous substance misidentified as harmful, the immune system produces antibodies called immunoglobulins, or Igs. The Ig responsible for allergic reactions is IgE. These IgE molecules attach themselves to mast cells (a type of cell containing histamine and other allergy mediators instrumental in the allergic response).
The second time the person encounters the substance, the IgE antibodies bound to the mast cells recognize surface markers of the allergen (antigens). As a consequence, the mast cells become activated.
Activated mast cells release powerful chemical mediators such as histamines, prostanoids, leukotrienes, chemokines, and cytokines from preformed granules. In addition, the activation of mast cells leads to the continued production of these mediators, promoting long-term effects.
The released mediators activate other leukocytes, including eosinophils and Th2 lymphocytes, and recruit them to the site of inflammation.
These cells promote downstream effects including eosinophilia and further IgE production. This late response can easily convert into a chronic inflammatory response.