Eosinophils are a type of white blood cell that plays a specialized role in the body’s immune system. They are typically measured as part of a complete blood count (CBC) with a differential, which determines the proportions of different white blood cell types in a blood sample. A count outside the normal range can offer clues about an underlying health condition, helping medical professionals monitor the body’s response to challenges like allergies or infections.
The Role of Eosinophils in the Body
Eosinophils defend the body against parasitic infections and allergens. These cells are produced in the bone marrow and circulate in the bloodstream before migrating into tissues, particularly those lining the gastrointestinal and respiratory tracts. Eosinophils contain granules filled with toxic proteins, such as Major Basic Protein (MBP) and Eosinophil Cationic Protein (ECP). When the body encounters a large parasite, such as a helminth (worm), eosinophils release these cytotoxic proteins to destroy the invader. Eosinophils are also involved in allergic and asthmatic reactions, accumulating at the site of inflammation, and contributing to symptoms like hay fever and asthma by releasing mediators that cause tissue damage and swelling.
Determining the Normal Percentage and Absolute Count
Eosinophils are measured in two ways: as a percentage of the total white blood cell (WBC) count and as an absolute count. In healthy individuals, eosinophils typically make up less than 5% to 7% of the total circulating white blood cells. The absolute eosinophil count (AEC) is considered more medically significant than the percentage, as it reflects the actual number of cells per volume of blood. The AEC is calculated by multiplying the eosinophil percentage by the total WBC count, and a typical normal AEC for an adult is less than 500 cells per microliter (cells/μL) of blood. The absolute count is important because a percentage can be misleading if the overall WBC count is abnormal, such as when a normal percentage still results in a low absolute count because the total WBC count is very low. Reference ranges can vary slightly between laboratories, so results should be interpreted using the specific range provided on the test report.
Causes of High Eosinophil Counts
An elevated eosinophil count, known as eosinophilia, usually points to an underlying medical issue, as these cells increase in response to a stimulus. The most common cause is an allergic condition, including asthma, allergic rhinitis (hay fever), eczema, and food allergies, which trigger the immune system to recruit eosinophils. Parasitic infections, especially tissue-invading helminths, are another major cause of eosinophilia, as the body uses these cells to combat large organisms. Eosinophilia can also result from a hypersensitivity reaction to certain medications. Less common, but more serious, causes include certain autoimmune diseases and specific types of cancers. Conditions like Hodgkin’s lymphoma, some leukemias, and chronic inflammatory disorders such as eosinophilic esophagitis can lead to persistently high levels. When the count exceeds 5,000 cells/μL and no clear cause is found, it may be classified as hypereosinophilic syndrome, which can damage organs like the heart, lungs, and nervous system.
Significance of Low Eosinophil Counts
A low eosinophil count, termed eosinopenia, is less frequently a cause for concern than a high count. Eosinopenia often occurs temporarily in response to acute physical or emotional stress. During stress, the body releases elevated levels of the hormone cortisol, which suppresses the production and activity of eosinophils. The use of corticosteroid medications, such as prednisone, is a well-known cause of eosinopenia, as these drugs mimic cortisol and actively reduce circulating eosinophils. A low count can also be observed during the early stages of some acute bacterial infections, where neutrophils dominate the immune response. A single instance of a low eosinophil count is generally not considered a major threat to overall health. However, a persistently low count, especially with other abnormal blood cell levels, may prompt investigation to rule out conditions such as Cushing’s syndrome or a problem with bone marrow function.