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Whatever are eosinophils?

Eosinophils are a type of white blood cell (corpuscle) and take up the red dye eosin when blood is examined under a microscope by the commonest method.

They accumulate wherever allergic reactions like those in asthma take place. Their natural role is to defend us against parasites. In fact allergies such as asthma are probably a malfunction of our protective mechanism against parasites.

Diagram of eosinophil as seen under the microscope after staining a blood smear with the red dye eosin, which stains the granules in the cytoplasm and with haematoxylin, which stains the nucleus blue. In the body all these things are colourless, of course. The nucleus consists of two lobes. The red-stained granules contain toxic proteins, ready for secretion from the cell.

The toxins from the granules are important for killing parasites, but in asthma they are released inappropriately and damage the lining of the air passages.

It is one of the objectives of asthma treatment to stop eosinophils from accumulating in your lungs and to stop those already there from causing damage. Steroid inhalers have a key role in doing this.

In normal blood, eosinophils amount to about 0 to 3 percent of the white blood cells, but this is not such a good guide because variation in the number of other cells alters this figure. A figure of 0 percent normally just means that there were no eosinophils among the limited number of white blood cells examined by the technician, and this is quite normal. If the counting is done by machine, eosinophils are normally not counted at all, perhaps giving the false impression that there are none. It is better to express the number of eosinophils in blood as the number in a unit of volume. The normal range is about 0.04 to 0.4 eosinophils x 10^9 per litre in the UK, or 40,000,000 to 400,000,000 per litre, or 40 to 400 per cubic millimetre (microlitre).
The term 'reference range', often used instead of 'normal range', means almost the same as as far as ordinary folk are concerned. For purists and nit-pickers, it avoids making hard-to-justify statements about what is normal, and quite correctly just relates results to the pattern seen in a 'reference population'. Reference ranges will vary from population to population. They may even vary between labs, because of population sampling issues and because no human activity is ever perfectly standardised.