Eosinophils are white blood cells specialised in combating parasitic infections and mediating allergic and inflammatory responses. They are recruited to sites of allergic inflammation — such as the airways in asthma or the skin in eczema — where they release toxic granule proteins and inflammatory mediators.

Elevated eosinophils (eosinophilia) most commonly indicate allergic disease (hay fever, asthma, food allergy, eczema) or parasitic infection — particularly intestinal worms. Significantly elevated eosinophils can also indicate hypereosinophilic syndrome, eosinophilic gastrointestinal disease, certain drugs reactions, or vasculitis. Eosinophil count is an important component of the allergy workup and is reported as part of the full blood count differential.