• Parasitic defense: Eosinophil proliferation and activation during multicellular parasitic infection is stimulated by IL-5 produced by TH2 and mast cells (not to be confused with IL-4, which stimulates IgE production). When a parasite invades the mucosa or enters the bloodstream, it is coated by IgG and IgA antibodies that bind the Fc receptors located on the eosinophil cell surface. This triggers eosinophil degranulation and release of cytotoxic proteins (eg, major basic protein) and reactive oxygen intermediates, substances that damage and destroy antibody-bound parasites. This mechanism is an example of antibody-dependent cell-mediated cytotoxicity (ADCC), which is also used by macrophages, neutrophils, and natural killer cells.
• Type I hypersensitivity reactions: Eosinophils also synthesize prostaglandins, leukotrienes, and cytokines that contribute to the inflammation seen in late-phase type 1 hypersensitivity and chronic allergic reactions.