B cells specific for allergens are activated by TH2 and TFH cells, as in other T cell-dependent B cell responses (see Chapter 11). Under the influence of CD40 ligand and cytokines, mainly IL-4, produced by these helper T cells, the B cells undergo heavy chain isotype switching and produce IgE. IgE circulates as a bivalent antibody and is normally present in plasma at a concentration of less than 1 |g/mL. In pathologic conditions such as helminthic infections and severe atopy, this level can rise to more than 1000 |g/mL. Allergen-specific IgE produced by plasmablasts and plasma cells enters the circulation and binds to Fc receptors on tissue mast cells, so that these cells are sensitized and poised to react to a subsequent encounter with the allergen. Circulating basophils are also capable of binding IgE.
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