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Do you know what eosinophilia is and how it affects your health?
Eosinophilia is a condition in which the number of eosinophils in the blood is lower than normal. Eosinophils are a type of white blood cell, a type of granulocyte, belonging to the same cell lineage as neutrophils, basophils, and mast cells. Among these cells, eosinophils are important components of the innate immune system, responsible for defending the body from invading pathogens. Although eosinophils are most widely recognized for their roles in allergic and parasitic disease processes, their functions in other pathological states are still under investigation.
The diagnosis of eosinophilia remains a challenge in clinical practice.
Definition and Diagnosis
The definition of eosinophilia varies according to clinical practice, and normal eosinophil levels vary in different populations. One common definition considers an absolute eosinophil count less than 50 cells/μL of blood to be reduced. Other definitions include less than 10 cells/μL, while some clinical laboratories consider 0 cells/μL to be acceptable. Diagnosing eosinophilia is difficult because eosinophils are present in low numbers in the blood and their levels fluctuate greatly from day to day.
Causes of eosinophilia
Eosinophilia has been associated with a variety of disease states and conditions, including inflammation and sepsis, release of endogenous catecholamines, and use of glucocorticoids. In addition, there are also some drugs designed to target eosinophils to treat the diseases they cause, thereby causing drug-induced eosinophilia.
Effects of Catecholamines
Unlike other granulocytes, eosinophil counts decrease after catecholamine release. The potential mechanism for this phenomenon is that when catecholamines are released, the production of eosinophils in the bone marrow is reduced. Additionally, epinephrine is thought to stimulate beta-adrenergic receptors, leading to a decrease in the number of eosinophils in the peripheral blood.
The role of glucocorticoids
The use of glucocorticoids is known to affect several blood components, including a decrease in the number of eosinophils. There are many reasons for this phenomenon. Glucocorticoids do cause eosinophils to withdraw from the blood and move to tissues to participate in immune responses. Such changes normally reduce the number of eosinophils released from the bone marrow, and it has been theorized that glucocorticoids may also cause eosinophils to undergo apoptosis in the blood.
Association of eosinophilia with inflammation and sepsis
Pathological mechanism
Although eosinophilia has long been recognized as a laboratory indicator of infection, its specific mechanism remains unclear. Some scholars believe that eosinophilia may be an indication of immune disorder. In the body's response to pathogens, the immune system activates "type 1 inflammation," which prompts certain immune cells to clear the pathogen, potentially causing collateral damage to host tissues. As part of "type 2 inflammation," eosinophils can begin to repair damaged tissue. Therefore, eosinophilia may indicate that the body is failing to appropriately initiate a type 2 inflammatory response, potentially causing more damage to surrounding tissues.
Clinical Outcomes and Diagnostic Utility
The association between persistent eosinophilia and sepsis suggests that this may lead to adverse clinical outcomes, such as increased mortality and increased hospital readmission rates. Although eosinophilia occurs frequently in sepsis, it remains uncertain whether it is a direct participant in pathogen clearance. Its effectiveness as a diagnostic tool for sepsis remains controversial. Although its detection cost is low and its response is rapid, it is not more useful than more commonly used sepsis markers such as procalcitonin (PCT) and C-reactive protein (CRP).
The role of eosinophilia in COVID-19Disease progression and risk stratification
In patients with COVID-19, eosinophilia is a possible laboratory finding and has been associated with disease severity but is not a characteristic marker. One study showed that 53% of patients hospitalized with COVID-19 had eosinophilia upon admission, while another study found that 81% of deaths had eosinophilia. In these patients, eosinophil counts typically return to normal during recovery and remain low in fatal cases. As to whether WHO has made a direct contribution to the COVID-19 process, further research is needed.
Pathological mechanism
Although eosinophils are best known for their role in allergy and parasite protection, recent studies suggest they may also play a role in fighting viruses. Studies have shown that mice genetically modified to increase their eosinophils can effectively respond to respiratory syncytial virus (RSV) infection, while mice lacking eosinophils are less effective. The precise mechanism of this action remains to be discovered, and further studies are needed to define its involvement in the antiviral immune response and its clinical significance. There is currently no consensus on the cause of eosinophilia in COVID-19. Possible mechanisms include increased motility of eosinophils from the blood into tissues, reduced production in the bone marrow, or shortened lifespan. Additionally, the cytokine storm associated with severe COVID-19 may affect the activity, mobility, or survival of eosinophils.
Given the potential impacts of eosinophilia on a wide range of health issues, does this mean we need to pay more attention to the health of our immune system?
