EosinophilsEosinophil count
White cells associated with allergy and parasitic infection.
Eosinophils are a small subset of white blood cells that release substances toxic to parasites and take part in allergic inflammation.
It matters because their count is normally low, so a rise is comparatively informative: allergy, asthma, eczema, drug reactions and parasitic infection are the usual reasons.
In practice the count varies through the day and is suppressed by steroid medication, so a single value is read in context alongside symptoms.