eosinophil
Summary
Eosinophils are granulocytic white blood cells involved in defense against parasitic infections and mediation of allergic reactions. They are identified histologically by bright red-orange cytoplasmic granules on H&E stain and typically comprise 1-6% of circulating leukocytes. Elevated counts (eosinophilia) are classically associated with parasitic infections, allergic conditions, and certain neoplasms.
Detail
Eosinophils are bilobed granulocytes derived from myeloid progenitor cells in the bone marrow, differentiation driven primarily by IL-5, IL-3, and GM-CSF. Their granules contain major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil peroxidase, and eosinophil-derived neurotoxin—these are cytotoxic to helminths but also contribute to tissue damage in allergic/inflammatory conditions (e.g., damaging bronchial epithelium in asthma).
Key clinical associations remembered by mnemonic 'NAACP': Neoplasia (Hodgkin lymphoma, eosinophilic leukemia), Allergic processes (asthma, atopic dermatitis, allergic rhinitis), Addison disease, Collagen vascular diseases (eosinophilic granulomatosis with polyangiitis/Churg-Strauss), Parasites (invasive helminth infections, not simple gut colonization). Drug reactions (DRESS syndrome) also cause eosinophilia.
Eosinophils play a role in the late-phase allergic reaction, recruited via IL-5 and eotaxin, and are targeted therapeutically by biologics such as mepolizumab, reslizumab (anti-IL-5), and benralizumab (anti-IL-5 receptor) used in severe eosinophilic asthma. They are also found in elevated numbers in eosinophilic esophagitis (>15 eos/hpf on biopsy). Charcot-Leyden crystals, formed from eosinophil breakdown products (galectin-10), are seen in sputum of asthmatics and stool in parasitic infections.
Corticosteroids cause eosinopenia by inducing apoptosis and altering trafficking, which is why eosinophil counts drop in Cushing syndrome or with steroid administration.
Sources
- First Aid for the USMLE Step 1
- Robbins and Cotran Pathologic Basis of Disease
- Kaplan USMLE Step 1 Lecture Notes: Immunology
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