The discovery of an elevated white blood cell count on a blood test can be a source of significant worry, especially when the term “eosinophil” is unfamiliar. These cells are a standard component of the immune system, and their high concentration, a condition called eosinophilia, is usually a response to a common and non-life-threatening event. While media coverage and online searches often link abnormal blood results to serious illnesses, a high eosinophil count is most frequently an indicator of the body actively fighting a less severe issue.
What Are Eosinophils and Eosinophilia?
Eosinophils are a type of white blood cell, or leukocyte, that circulates in the blood and resides in various tissues like the gastrointestinal tract, skin, and lungs. Their primary biological function is to defend the body against multicellular parasites, such as hookworms, by releasing toxic granule contents that destroy the invader. Eosinophils also play a significant role in allergic and inflammatory responses, where they contribute to the localized tissue damage seen in conditions like asthma.
The count of these cells is measured as the Absolute Eosinophil Count (AEC) in cells per microliter of blood. In healthy adults, the AEC typically ranges from 0 to 500 cells/µL. Eosinophilia is formally defined when the count exceeds 500 cells/µL.
For clinical assessment, the severity of the elevation is categorized based on the absolute number. Mild eosinophilia falls between 500 and 1,500 cells/µL, and is the most common finding on routine blood work. Moderate eosinophilia is counted between 1,500 and 5,000 cells/µL, while severe eosinophilia is defined as a count greater than 5,000 cells/µL, which prompts a more urgent investigation.
Common Non-Malignant Causes of High Eosinophils
The vast majority of high eosinophil counts are reactive, meaning they are a temporary response to a stimulus that is not cancer. Worldwide, the most frequent cause of significant eosinophilia is parasitic infection, particularly by helminths, or parasitic worms. These infections, often acquired through contaminated food, water, or soil, trigger a powerful immune mobilization of eosinophils to the site of invasion.
Allergic conditions are the most common cause of mild to moderate eosinophilia in developed countries. This includes common disorders like seasonal allergies (hay fever), asthma, and eczema. In these cases, the immune system mistakenly targets a harmless substance, like pollen or a food protein, with an inflammatory response that involves high numbers of eosinophils.
A number of medications can also provoke a hypersensitivity reaction that results in eosinophilia, sometimes without obvious symptoms. Drugs like certain antibiotics, non-steroidal anti-inflammatory drugs (NSAIDs), and some anti-seizure medications are known to be triggers. Resolving this type of eosinophilia often requires identifying and discontinuing the offending drug.
Specific inflammatory disorders are another non-malignant cause, including some autoimmune conditions and eosinophilic gastrointestinal diseases. For example, eosinophilic esophagitis involves a dense infiltration of eosinophils into the esophagus, causing difficulty swallowing.
The Connection to Cancer and Malignancy
While cancer is a concern, it represents a relatively rare cause of an elevated eosinophil count compared to allergies or parasites. When malignancy is the underlying reason, the eosinophilia falls into one of two general categories. The first is secondary eosinophilia, also known as a paraneoplastic syndrome, where a tumor stimulates the production of eosinophils.
Certain solid tumors, such as lung, colorectal, and ovarian cancers, can release signaling molecules, notably Interleukin-5 (IL-5). IL-5 travels to the bone marrow and encourages the production and release of eosinophils. Paraneoplastic eosinophilia is also associated with certain hematologic cancers, like Hodgkin lymphoma and specific types of T-cell lymphomas. In these instances, the high count is a side effect of the cancer rather than the cancer itself.
The second category is primary eosinophilia, where the eosinophils themselves are part of the malignant process. This is seen in rare blood cancers like Eosinophilic Leukemia or certain forms of Hypereosinophilic Syndrome (HES). In these conditions, the eosinophil precursor cells in the bone marrow have an acquired genetic abnormality that causes them to proliferate uncontrollably.
No specific eosinophil level definitively indicates cancer. The focus is always on whether the elevation is persistent and cannot be explained by more common causes. The presence of paraneoplastic eosinophilia in solid tumors is often a sign of more advanced or aggressive disease, necessitating a thorough workup when common causes are ruled out.
Diagnostic Process and Next Steps
A clinician follows a systematic process to identify the cause of an elevated eosinophil count. The initial step is a comprehensive patient history and physical examination, focusing on recent travel to endemic areas for parasites, current medications, and any personal or family history of allergies. This initial information can often pinpoint a reactive cause immediately.
Laboratory investigation confirms the absolute count and looks for signs of immune activation, such as elevated IgE levels, which support an allergic cause. A stool sample analysis for ova and parasites is a routine test used to rule out helminth infection, especially if travel history is relevant. The next steps are guided by the severity and persistence of the elevation and the patient’s symptoms.
If common causes are excluded and eosinophilia is persistent or severe, the focus shifts to less common inflammatory disorders or primary hematologic conditions. This may involve specialized blood tests to look for specific genetic markers, like the FIP1L1-PDGFRA fusion gene associated with some HES cases. A bone marrow biopsy might be performed to assess the bone marrow for signs of primary blood cancer or excessive eosinophil production.