Eosinophils are white blood cells in the innate immune system, important in the defense against parasites (esp. worms/helminths). They are recruited and stimulated by Interleukin-5 (IL-5), and activated by antibodies bound to parasitic antigens.  Once activated, eosinophils degranulate, releasing Major Basic Protein (MBP). MBP is toxic to all cells, but is extremely toxic to parasitic worms/helminths. Aberrant eosinophilic activation contributes to the pathogenesis of allergy, asthma, and certain vasculitides such as Churg-Strauss syndrome. In these autoimmune diseases, corticosteroids can be administered, since they block the activation of eosinophils.

Key Points

  • Eosinophil
    • Cell of the innate immune system 
    • Contains bilobate nucleus and eosinophilic granules 
      • Granules stain pink (eosinophilic) on H&E
    • Recruited and stimulated by Interleukin-5 (IL-5)
      • Also by eosinophil chemotactic factors (e.g. C3a and C5a)
    • Activated by Fc regions of antibodies (e.g. IgE and IgG)
      • Releases Major Basic Protein (MBP) when activated
        • Extremely toxic to parasitic worms
        • Can damage normal tissue, contributing to allergy and asthma
          • Charcot-Leyden crystals in asthma are formed by degraded eosinophils
        • Aberrant activation can lead to vasculitis (e.g. Churg-Strauss Disease)
    • Inhibited by corticosteroids