Eosinophils

Eosinophils are made for an enemy too large to swallow: worms and other parasites. They attach to the surface and pour enzymes and toxic proteins over it. Those same granules, which stain pink under the microscope and give the cell its name, are also released with allergy. With hay fever, asthma and eczema the immune system sends eosinophils to the nose, the airways and the skin, and the count in your blood rises along. They are scarce, a few percent of your white cells, and the largest part sits in the tissue, especially in the gut wall. The count in your blood follows the stimulus. In the pollen season it sits higher in anyone with hay fever than in winter. A medicine your body reacts to, some antibiotics, painkillers and drugs for epilepsy, lifts the count, as does a worm infection you have brought back from the tropics. And cortisol suppresses it: with acute stress, a severe infection and during a course of prednisone the count falls to almost zero, which is the reason prednisone works so well for asthma and eczema. Your report shows the count in ×10⁹/L, and read it as an absolute count, not as a percentage. Normal is about 0.1 to 0.4 ×10⁹/L, where the labs overlap. The lower end says nothing; it is the upper end that means something.

Eosinophils are the number that makes allergy visible. Anyone who is tired in spring, has a blocked nose and wonders whether it is a cold or hay fever sees the answer in the differential: with hay fever the eosinophils rise, with a cold the lymphocytes. In asthma the count is moreover a measure of the type: asthma with many eosinophils responds well to inhaled steroids and to new drugs that switch off exactly this cell, and the count in your blood helps with that choice. A second reason is that a high count without allergy is a clue to something you would otherwise miss. A worm infection after a trip often gives no symptoms for months except a slightly raised eosinophil count. A medicine your body reacts to lets the count rise before the rash comes. And the rare cases, an autoimmune disease of the blood vessels or a bone marrow disease in which production goes astray, begin with a count that keeps rising without explanation. A low count is the side you can ignore. Eosinophils are naturally scarce, three of the four labs start their band at zero, and cortisol suppresses the count with every form of stress or illness. A count of zero mainly says that no allergy or parasite is at work at that moment.

A low count means nothing. Cortisol suppresses eosinophils with acute stress, a severe infection or during a course of prednisone, and an inhaled steroid for asthma also suppresses the count a little. There is no lower limit that points to a deficiency or a condition. A high count means there is a stimulus, and the question is which. From common to rare: an allergy, in the Netherlands usually hay fever, asthma or eczema, and then the count tracks the season and the symptoms; a medicine your body reacts to, often in the first weeks after starting; and a parasite, especially after a trip to the tropics or with gut symptoms. Rare are an autoimmune disease of the blood vessels and a bone marrow disease, and you recognise those by a count that repeatedly stays far above the band, often above 1.5, without allergy or travel as explanation. Such a count should be investigated, even without symptoms, because eosinophils that stay high for long can damage the heart and the lungs. Set eosinophils beside the total of white cells and the other types, because an allergy lifts only this cell and an infection the others, and beside CRP, which stays normal with an allergy.