Eosinophils are granulocytic white blood cells characterised by prominent cytoplasmic granules that stain strongly with eosin dyes, giving them their name. They are particularly associated with immune responses against helminth parasites and with allergic and eosinophil-associated inflammatory conditions. Eosinophils develop in the bone marrow and circulate in blood before migrating into tissues, especially mucosal tissues such as the gastrointestinal and respiratory tracts. Their functions include release of granule proteins, production of lipid mediators and cytokines, interaction with antibodies and participation in tissue immune regulation.
Eosinophil granules contain highly charged proteins with potent biological effects. Major basic protein, eosinophil peroxidase, eosinophil cationic protein and eosinophil-derived neurotoxin are among the best-known granule constituents. These substances can damage parasites and contribute to antimicrobial defence, but they can also injure host epithelial tissues when eosinophil activation is excessive. Eosinophils therefore illustrate the dual nature of immune effector mechanisms: substances effective against large organisms can also produce pathological inflammation when released inappropriately or persistently.
Eosinophils are strongly associated with defence against helminths because large parasites cannot generally be phagocytosed in the same manner as small bacteria. Antibody and complement-related mechanisms can facilitate recognition of parasite surfaces, while eosinophils can attach to antibody-coated targets through Fc receptors and release toxic granule contents. Type 2 immune responses involving cytokines such as interleukin-4, interleukin-5 and interleukin-13 contribute to eosinophil development, recruitment and activation. Eosinophils can therefore participate in a coordinated response involving B Cells, Antibodies and T-cell subsets.
Eosinophils have important roles in allergic disease. Their numbers and activity can increase in conditions such as eosinophilic asthma and some forms of allergic inflammation. Eosinophil-derived mediators can contribute to epithelial injury, mucus changes and persistent inflammation. However, eosinophils are not themselves synonymous with allergy, because they also have physiological functions and participate in parasite defence and tissue immune surveillance. Similarly, an elevated eosinophil count can have many causes, so eosinophilia requires interpretation in clinical context rather than being treated as proof of an allergic disorder.
Eosinophil recruitment depends heavily on chemokine and cytokine signalling. Eotaxins are chemokines strongly associated with eosinophil recruitment through the CCR3 receptor, while interleukin-5 is particularly important for eosinophil development, survival and activation. Eosinophils can therefore accumulate at sites where the local cytokine environment supports their persistence. Once recruited, they can interact with epithelial cells, mast cells, lymphocytes and other leukocytes. The resulting response can be protective or pathological depending on the antigen, duration and regulatory mechanisms present in the tissue.
Eosinophils are distinct from Neutrophils despite both being granulocytes. Neutrophils are generally dominant in acute bacterial and fungal defence and are specialised for rapid phagocytic and oxidative killing, whereas eosinophils are more strongly associated with helminth defence and type 2 inflammation. Their granules also contain different molecular contents and have different staining characteristics. Eosinophils can participate in some phagocytic functions, but their major significance lies in extracellular effector mechanisms and immunomodulation rather than functioning as the principal rapid phagocytes of the bloodstream.
Eosinophil counts can be assessed through a full blood count with differential measurement. Eosinophilia may occur in parasitic infections, allergic disease, certain drug reactions, some autoimmune or inflammatory conditions and particular haematological disorders. Interpretation requires attention to the absolute eosinophil count, persistence, symptoms, travel or exposure history, medication use and other laboratory findings. A transient mild increase can have a different significance from persistent marked eosinophilia accompanied by organ involvement. Therefore, the numerical count alone cannot establish the underlying cause.
Therapeutic approaches to eosinophil-associated disease have increasingly targeted specific biological pathways. Corticosteroids can suppress eosinophilic inflammation, while biologic medicines targeting molecules involved in type 2 inflammation or eosinophil survival can be used in selected conditions. Such therapies demonstrate the clinical importance of understanding eosinophil development and recruitment. Eosinophils are neither universally harmful nor universally beneficial: they provide important defence mechanisms while also having the capacity to contribute substantially to tissue injury when their activation becomes excessive, prolonged or misdirected.
