The IgE-mediated immune response defends against parasites and toxins with which two steps?

• A. Sensitization and subsequent exposure
• B. Activation and clonal expansion
• C. Antibody production and phagocytosis
• D. Inflammation and recruitment

Answer: (A)

IgE-mediated defense relies on a priming step followed by an actual trigger that activates the effector response.

First, during sensitization, exposure to the parasite or relevant antigen leads B cells to switch to producing IgE specific for that antigen. These IgE molecules bind to FcεRI receptors on mast cells and basophils, arming them without causing immediate release.

Then, at subsequent exposure to the same antigen, the bound IgE on these effector cells is cross-linked by the antigen. This cross-linking prompts rapid degranulation and production of mediators such as histamine, leukotrienes, and prostaglandins, along with recruitment of eosinophils. The result is inflammation, increased mucus and vascular permeability, and mechanisms that help expel parasites.

Other options describe parts of immune activation or downstream effects, but they don’t capture the two-step priming and trigger sequence that defines the IgE-mediated response.