major basic protein

Major basic protein is a core component of eosinophilic granules, synthesized and stored in these cells and released upon eosinophil activation. It is highly cationic (basic) and exerts its cytotoxic effects by directly damaging the cell membranes of helminths and other parasites, making it a critical effector molecule in anti-parasitic immunity. Eosinophils are recruited to sites of parasitic infection via IL-5 (which promotes eosinophil production and activation) and chemokines like eotaxin, and upon degranulation release MBP, eosinophil cationic protein (ECP), eosinophil peroxidase, and eosinophil-derived neurotoxin.

While protective against parasites, MBP is also implicated in the pathogenesis of allergic and atopic diseases, particularly asthma. In the airways, MBP causes epithelial cell damage and desquamation, contributing to bronchial hyperreactivity, mucus plugging, and the chronic inflammatory changes seen in asthmatic airway remodeling. This same protein found in nasal polyps and sputum of asthmatic patients can be used as a marker of eosinophilic inflammation.

Clinically, elevated eosinophil counts (eosinophilia) should prompt consideration of the mnemonic NAACP: Neoplasia, Asthma, Allergic processes, Collagen vascular disease, and Parasites (specifically invasive helminths, since intraluminal parasites like Enterobius don't typically cause eosinophilia). MBP and eosinophil activity are also relevant in hypereosinophilic syndrome and eosinophilic esophagitis. Understanding this molecule is important for correlating basic immunology with clinical presentations of allergy, asthma, and parasitic disease on USMLE exams.