Red blood cells(RBC's) can vary in size due to pathologies but for the most part are consistently 7.5-8 micrometers in diameter. Since RBC's are abundant in any typical blood smear, they can be used as visual measurement tools for the leukocytes or WBC's one encounters. Compare the diameter of any WBC in question to a nearby RBC to gain an estimate of the size for the former. Sizes of cells, in combination with the other criteria, granulation/color, nuclear shape/density, is very helpful in the final identification of a leukocyte type. Typical cell diameters for leukocytes are:
Using the RBC as a small ruler now, go back to the image and estimate the sizes of the WBC's to aid in your identification.
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