Which leukocyte is most commonly associated with allergic reactions and parasitic infections?

• A. Basophil
• B. Eosinophil
• C. Neutrophil
• D. Lymphocyte

Answer: (B)

Eosinophils are the white blood cells most closely associated with allergic reactions and defense against parasites. They respond to parasites such as worms by releasing toxic granule contents, including major basic protein, to damage or kill the parasite. In allergies, a Th2-driven response elevates eosinophils (via IL-5) and they contribute to inflammation and tissue effects seen in conditions like asthma. This combination—parasite-targeted cytotoxicity and involvement in allergic inflammation—makes eosinophils the best answer. Basophils primarily drive immediate hypersensitivity via histamine release, neutrophils handle bacterial infections, and lymphocytes are central to adaptive immunity, not specifically parasitic defense or allergic responses.