Eosinophils · What Are They, What Do They Do, and More

Eosinophils play a role in the immune system by helping fight infections and producing inflammatory responses. Under normal conditions, the absolute eosinophil count (i.e., the number of eosinophils circulating in the body) is relatively low, usually around 1-4% of the total white blood cell count. However, when the immune system is activated, the number of eosinophils released increases, and they flock to the site of the infection to help combat the infectious agent or incite an inflammatory reaction.  

When activated, eosinophils release their granulocyte content - highly toxic proteins and free radicals - to help target microorganisms, especially parasitic infections. These same proteins and radicals can cause tissue damage when eosinophils are inappropriately activated during allergic reactions. Activated eosinophils also release chemical mediators like prostaglandins, cytokines, and leukotrienes, which assist in boosting the inflammatory response.  

High levels of eosinophils, also known as eosinophilia, are usually a result of allergic reactions, including asthma, allergic rhinitis, and atopic dermatitis; parasitic infections; and certain cancers, like Hodgkin lymphoma and leukemia.  

Sometimes, when there are high levels of eosinophils, they can exit the bloodstream and accumulate in other organs and tissues, which can cause eosinophilic disorders. Symptoms may occur depending on where the eosinophils accumulate. Some examples of eosinophilic disorders include eosinophilic cardiomyopathy, which can manifest with chest pain and shortness of breath; eosinophilic colitis, which can present as diarrhea; eosinophilic esophagitis, which involves difficulty swallowing; and eosinophilic pneumonia, which can manifest as cough and shortness of breath.  

In rare cases, eosinophils increase to alarming levels and the individual can develop hypereosinophilic syndrome (HES). This syndrome is diagnosed when blood tests reveal persistently high levels of eosinophils for a period of at least 6 months.  

Finally, eosinophilia can also be caused by a rare condition called eosinophilic granulomatosis with polyangiitis. This condition is characterized by high levels of eosinophils along with asthma and inflammation of the blood vessels (i.e., vasculitis).