Eosinophils are strongly reactive to the dye eosin in smears, hence the name. They stain a bright orange-pink color. Typically the nucleus is bi-lobed, not multi-lobed. Their function isn't completely understood, but it's known that eosinophils are involved in allergic responses. The eosinophil's granules contain lytic enzymes. Eosinophils have been proposed to engage in phagocytosis, but not of particulate antigens: rather to engulf and destroy the precipitated antigen-antibody complexes produced in humorally based immune reactions.

It's known that the eosinophil counters the action of many mast cell mediators by secreting degradative enzymes, including histaminase and aryl sulphatase, which destroy histamine and leukotrienes respectively.  An elevated eosinophil count is typical in allergic reactions. Eosinophils chemotactically aggregate in large numbers to sites where antigen-antibody complexes are found and where the host is attempting to deal with foreign bodies. They're found easily in the connective tissue below a wheal, the raised "bump" that results at the site of an insect bite. The strong immune response such a bite elicits brings them in, and a section taken through a wheal will have thousands of eosinophils evident.