Eosinophils evolved to fight worms, yet they also drive asthma
Eosinophils make up only 1–3% of white blood cells, but their granules carry a toxic arsenal built for killing multicellular parasites. That same weaponry damages our own tissue when the cells gather where they don't belong, such as the airways of people with eosinophilic asthma, where their numbers track how severe attacks get.
The name means acid-loving. Under the Romanowsky staining method, the red dye eosin turns their otherwise transparent granules brick red. Those granules hold enzymes and proteins including eosinophil peroxidase and major basic protein, released when the cell is activated in a process called degranulation. Each cell measures roughly 12–17 micrometres and usually has a two-lobed nucleus. Born in the bone marrow under the direction of the signalling molecule IL-5, they circulate in blood for only 8–12 hours before settling into tissues, where they can last another 8–12 days.
Location matters. Healthy people carry eosinophils in the gut, thymus, spleen and lymph nodes, but not normally in the lungs, skin or oesophagus, and finding them there signals disease. Their granule proteins are harsh: one punches pores in target cell membranes, while the peroxidase generates reactive oxygen species that kill cells. Two of the proteins are ribonucleases with antiviral activity, hinting at a role against viruses as well as worms. The cells also turn up in mammary gland development, transplant rejection and tumours.
Counting them has practical value. Above 500 cells per microlitre of blood is called eosinophilia, and above 1,500 hypereosinophilia. Allergy is perhaps the commonest cause, but parasitic infection, autoimmune conditions, some cancers and drugs such as penicillin can all raise the count. Because counts are skewed, medians are more useful than averages: large studies put the healthy median around 100 cells per microlitre, with 400 marking the upper edge of normal. That means cutoffs of 150 or 300 used in asthma and COPD care sit inside the healthy range.
Treatments target the cells' supply lines. Corticosteroids quickly cut blood numbers, and antibodies such as mepolizumab block IL-5 so fewer eosinophils are made. Practice does not always follow the evidence: a Cleveland Clinic study of 688 COPD patients started on inhaled steroids between 2021 and 2023 found 86% were prescribed outside the eosinophil criteria of the GOLD guidelines, and only 15% had a baseline count.
Source: Eosinophil
