What Are Eosinophils on a Blood Test?

What it means

Eosinophils are a type of white blood cell. They contain granules that stain bright orange-red with a dye called eosin, which is how they got their name. They help the body respond to parasites such as worms, and they play a large part in allergic reactions and asthma.

Eosinophils are listed in the differential of a complete blood count (CBC). They normally make up only a small share of white cells, so the numbers on the report are small. The lab gives the count and flags it if it sits outside the lab's range, so your doctor can consider whether allergy, a medicine reaction or something else needs a closer look.

Eosinophilia is the word for a raised eosinophil count. Eosinophils are one kind of granulocyte, so some machines count them inside a "granulocytes" total, but they are a different cell from neutrophils and basophils.

Where you may see it

• The differential section of a CBC, shown as "EOS" or "EOS%"
• Allergy and asthma clinic letters
• Results for nasal or sputum samples, where eosinophils are sometimes counted outside the blood