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How is eosinophilia related to infection? Why is it a health warning sign?
Eosinophilia is a condition characterized by lower-than-normal numbers of eosinophils. These white blood cells are part of the immune system and are crucial for defense against pathogens. Although commonly associated with allergies and parasitic diseases, the role of eosinophils in other pathologies is the subject of ongoing research. Of course, there is no consistent clinical definition of eosinophilia, and normal eosinophil levels vary from person to person.
Normal variations in eosinophil counts can make diagnosing eosinophilia challenging.
Definition and diagnosis of eosinophilia
In clinical practice, eosinophilia is often defined as an absolute eosinophil count of less than 50 cells/μLiter, while other definitions may be less than 10 cells/μLiter. This suggests that the interpretation of the range may vary between laboratories. Therefore, diagnostic variability increases the difficulty in understanding this condition.
Causes of Eosinophilia
Eosinophilia is commonly associated with a variety of pathological conditions. For example, inflammation, sepsis, release of endogenous catecholamines, and the use of glucocorticoids can all lead to a decrease in eosinophil counts. Some drugs that target eosinophils are also used to intentionally lower their numbers to treat related disorders.
The release of catecholamines tends to cause a decrease in eosinophil counts, probably through a mechanism that includes reduced production in the bone marrow.
The role of eosinophils in inflammation and sepsis
Eosinophilia is often considered a laboratory sign of infection. Although its exact mechanism is not yet clear, it is theorized that it may be an indicator of immune dysregulation. In response to pathogens, the immune system initiates "type 1 inflammation," using certain immune cells to fight the pathogen, while eosinophils are an important component of "type 2 inflammation," helping to repair damaged tissue.
The impact of eosinophilia in sepsisGiven that persistent eosinophilia is associated with worse clinical outcomes, including increased mortality and hospital readmission rates, this condition has potential diagnostic value. Although eosinophilia is common in sepsis, its validity as a biomarker remains controversial.
In most cases, eosinophilia is not as diagnostic as other more commonly used sepsis markers such as PCT and CRP, but is often a faster and cheaper testing option.
Association between eosinophilia and COVID-19
In patients with COVID-19, eosinophilia is a possible laboratory marker and correlates with disease severity. The study showed that 53% of hospitalized COVID-19 patients showed eosinophilia upon admission, as did 81% of severe cases. Although a decrease in eosinophils is not always a direct cause of disease progression, its trend is considered an important factor in risk assessment.
Future Research Directions
Our understanding of eosinophilia and its role in various pathological conditions remains incomplete and requires further study. Particularly in the context of COVID-19, understanding the impact of this phenomenon on the disease process will be an important area for future research. In the meantime, given the diverse functions of eosinophils, future discoveries may change our understanding of their roles.
Against this backdrop, could eosinophilia reveal something deeper about our understanding of how the immune system works?
