Which cytokine is primarily responsible for eosinophil activation and mucosal immunity against parasites?

• A. IL-2
• B. IL-4
• C. IL-10
• D. IL-5

Answer: (D)

Eosinophils are key effectors against parasites at mucosal surfaces, and the cytokine that directly drives their development, recruitment, and activation is interleukin-5. IL-5 is produced mainly by Th2 cells (and ILC2s) and stimulates eosinophil progenitors in the bone marrow to proliferate and mature, while also increasing survival and promoting activation of eosinophils in tissues. This leads to degranulation and release of toxic proteins that help kill parasites. Other cytokines here have different roles: IL-4 promotes IgE class switching and Th2 differentiation, IL-2 mainly supports general T cell growth, and IL-10 dampens inflammation. Hence, IL-5 best accounts for eosinophil-driven mucosal immunity against parasites.