Eosinophils are a distinct type of white blood cell, representing a small but significant component of the body’s immune system. Under a microscope, they are recognizable by their bilobed nucleus and the presence of coarse, round granules within their cytoplasm that readily stain pink or red with acidic dyes like eosin. These cells are part of the innate immune response, which serves as the body’s immediate, non-specific line of defense against perceived threats. Eosinophils are produced in the bone marrow and circulate in the bloodstream, making up about 1-5% of circulating leukocytes in healthy individuals.
Defending Against Parasites
Eosinophils have an evolutionarily conserved role in combating parasitic infections, especially large multicellular parasites such as helminths, commonly known as worms. Unlike smaller pathogens, these large invaders cannot be effectively engulfed and destroyed by phagocytosis, a process where immune cells “eat” foreign particles. Instead, eosinophils counteract these parasites by releasing pre-formed toxic proteins stored within their granules.
These proteins include Major Basic Protein (MBP), Eosinophil Cationic Protein (ECP), Eosinophil Peroxidase (EPO), and Eosinophil-Derived Neurotoxin (EDN). When eosinophils encounter a helminth, they degranulate, releasing these cytotoxic substances directly onto the parasite’s surface. This direct application of toxic proteins helps to damage the parasite’s outer layers and disrupt its physiological functions, contributing to its eventual destruction or expulsion from the host.
The effectiveness of eosinophils against helminths is enhanced by the presence of specific antibodies or complement proteins that bind to the parasite’s surface, signaling the eosinophils for attack. This mechanism, known as antibody-dependent cellular cytotoxicity, allows eosinophils to target and kill helminth larvae in a coordinated immune response.
Role in Allergic Responses
Eosinophils play a significant, yet detrimental, role in allergic reactions and various inflammatory diseases. They are heavily involved in conditions such as asthma, allergic rhinitis (hay fever), atopic dermatitis (eczema), and certain food allergies like eosinophilic esophagitis. In these scenarios, eosinophils are recruited to sites of allergic inflammation, driven by specific chemical signals, which promote their survival and activation.
Upon activation, eosinophils release a variety of inflammatory mediators, including leukotrienes, prostaglandins, and cytokines. The release of these substances contributes to tissue damage and chronic inflammation, which are hallmarks of allergic conditions. For example, in asthma, these mediators lead to increased mucus production, airway hyperresponsiveness, and tissue remodeling in the airways.
This response is an overreaction of the immune system to otherwise harmless substances, such as pollen, dust mites, or certain foods. The prolonged presence and activity of eosinophils in affected tissues can exacerbate symptoms and contribute to the progression of these chronic inflammatory diseases.
Beyond Primary Functions
Beyond their well-known roles in parasitic defense and allergic reactions, eosinophils contribute to other physiological processes, including tissue remodeling and repair. These cells are present in healthy tissues throughout the body, such as the gastrointestinal tract, lungs, mammary glands, and thymus, suggesting broader functions beyond immune defense. They contribute to wound healing by releasing growth factors, which can promote tissue regeneration and angiogenesis.
Eosinophils also have a role in immune modulation, interacting with other immune cells and influencing adaptive immune responses. They can act as antigen-presenting cells, processing and displaying antigens to T cells, thereby influencing their differentiation and cytokine production. This interaction allows eosinophils to help coordinate the immune response, bridging innate and adaptive immunity.
Eosinophils can secrete various cytokines, which regulate other immune cells and amplify inflammatory responses. These multifaceted interactions highlight that eosinophils are not merely effector cells but also active participants in maintaining tissue homeostasis and influencing the broader immune landscape.
What Eosinophil Counts Reveal
Eosinophil levels in blood tests provide indicators for clinicians to understand underlying health conditions. A normal eosinophil count ranges from 30 to 350 cells per microliter of blood, which can vary by lab. An elevated count, a condition known as eosinophilia, is defined as more than 500 cells per microliter of blood.
Eosinophilia often indicates conditions like parasitic infections or allergic reactions. Certain autoimmune disorders, like eosinophilic granulomatosis with polyangiitis, and some types of cancer, including certain leukemias, can also lead to elevated eosinophil levels.
Conversely, a low eosinophil count, termed eosinopenia, is less common but can also indicate health issues. Eosinopenia is defined as less than 30 cells per microliter of blood. Potential causes include acute stress, infections (especially severe bacterial infections or sepsis), and the use of certain medications, such as corticosteroids. These counts reflect the functional state of eosinophils and guide medical professionals in diagnosing and managing various health conditions.