Eosinophils: The Good, The Bad, and The Unknown

Eosinophils in our Immunity

While Eosinophils account for less than 5% of our white blood cells, they pack quite a punch (Jackson et al., 2022). Most people have likely heard about eosinophils in terms of their role in allergies, but their original purpose is to fight parasites (Vieira et al., 2026). Eosinophils are a type of white blood cell we call granulocytes. They’re named because they store toxic granules which they use to fight and kill infections. But this is far from the only effect that these powerful cells have.

Eosinophils express heterogeneity, a fancy term which essentially means that they’re split into a bunch of different subgroups. Furthermore, we see differentiation in Eosinophils based on their location in the body. But if they’re so diverse, how do we classify them?

Eosinophil subtypes can be classified by their surface markers, essentially what molecules the cell expresses on its membrane. Most eosinophils express Siglec-8 and CCR-3 on their membranes, which distinguishes them from other common granulocytes. But the presence of other markers, in either low or high amounts, helps us categorize the function of these eosinophil subtypes. For example, a CD101^lo eosinophil (expresses low amounts of CD101) has been attributed to certain regulatory activities in neutrophil-mediated inflammation, while CD101^hi eosinophils appear to promote inflammation (Ruzic et al., 2026). So, if that’s the case, what subtypes should we be looking at?

Eosinophils and Their Role in Diseases

Chronic obstructive pulmonary disease (COPD) and asthma are two widespread respiratory inflammatory diseases. In about half of patients, we see elevated airway eosinophil levels. In asthma, studies have shown that CD62L^lo eosinophils decrease the level of asthma control for the patient (Ruzic et al., 2026).

Under chronic inflammation, eosinophils tend to migrate into nerves. When eosinophils degranulate, they release toxic molecules like free radical species. While they’re extremely effective at tearing apart pathogens, they are non-specific and damage surrounding tissue at the same time. Given their destructive nature, they can cause nerve damage while also recruiting more eosinophils in the process (Diny et al., 2017).

Eosinophils also play a role in autoimmunity. Eosinophils can bind to antibodies and trigger degranulation through a process called anti-body dependent cellular cytotoxicity. This process significantly increases their effectiveness against parasites, but as with many things, it has a downside. Autoantibodies, antibodies that bind to your own cells, can also trigger anti-body dependent cellular cytotoxicity, causing eosinophils to degranulate and kill your own cells (Diny et al., 2017).

Eosinophils in connective tissue.

The Unknown

While we know a lot about these powerful cells, such as their role in combatting parasites, pathogens, and certain cancers, there’s still a lot of nuances we don’t understand of their processes in humans. For example, recent research has suggested that eosinophils potentially have a role in metabolism. Particularly, the molecules they secrete may have an impact on how blood sugar is regulated and how fat is deposited (Wechsler et al., 2021).

Interestingly, eosinophil deficiency (eosinopenia) has yet to be seen in humans congenitally. Although, in rare cases, patients can acquire Good’s syndrome, in which eosinophil deficiency occurs. Despite this, it does not appear to be consistent with any clinical symptoms. As such, the lack of congenital eosinophil deficiency, especially in countries with high sanitation, may be a result of underreporting (Klion et al., 2020).

Though many other gaps in our knowledge exist, a widely debated question is the benefits that eosinophils provide to human physiology in parasite-free environments. There is much research to be done in this area and many things to be discovered.

Summary and Conclusion

Eosinophils are known to play an integral role in the innate immune system, particularly effective against parasites although they also defend against bacteria, viruses, fungal infections, and certain types of cancer. Despite their strengths, they are a common cause of respiratory hypersensitivity and contribute to autoimmune disease damage by destroying cells bound by autoantibodies. While we do have the broad picture of how eosinophils function, we have major gaps in our knowledge particularly in the nuances of human physiology and regulation.

References

Diny, N. L., Rose, N. R., & Čiháková, D. (2017). Eosinophils in Autoimmune Diseases. Frontiers in Immunology, 8. https://doi.org/10.3389/fimmu.2017.00484

Jackson, D. J., Akuthota, P., & Roufosse, F. (2022). Eosinophils and eosinophilic immune dysfunction in health and disease. European Respiratory Review : An Official Journal of the European Respiratory Society, 31(163). https://doi.org/10.1183/16000617.0150-2021

Klion, A. D., Ackerman, S. J., & Bochner, B. S. (2020). Contributions of Eosinophils to Human Health and Disease. Annual Review of Pathology: Mechanisms of Disease, 15(1), 179–209. https://doi.org/10.1146/annurev-pathmechdis-012419-032756

Ruzic, A., Trus, M., Sehmi, R., & Mukherjee, M. (2026). Understanding Eosinophil Heterogeneity: The Known and Unknown. Cells, 15(6), 564. https://doi.org/10.3390/cells15060564

Vieira, B. M., Xavier‐Elsas, P., Moura‐Neto, V., & Gaspar‐Elsas, M. I. C. (2026). Eosinophil Biology Today—A Primer for Basic and Clinical Investigators. Immunology. https://doi.org/10.1111/imm.70104

Wechsler, M. E., Munitz, A., Ackerman, S. J., Drake, M. G., Jackson, D. J., Wardlaw, A. J., Dougan, S. K., Berdnikovs, S., Schleich, F., Matucci, A., Chanez, P., Prazma, C. M., Howarth, P., Weller, P. F., & Merkel, P. A. (2021). Eosinophils in Health and Disease: A State-of-the-Art Review. Mayo Clinic Proceedings, 96(10), 2694–2707. https://doi.org/10.1016/j.mayocp.2021.04.025

Assessed and Endorsed by the MedReport Medical Review Board