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Tortora writes in Principles of Anatomy and Physiology:

Lymphocytes may be as small as 6–9 μm in diameter or as large as 10–14 μm in diameter.

Those ranges are quite close to each others. Should the above be taken to mean that lymphocytes sizes are clustered in two groups, or is it just a way of saying that lymphocytes are 6-14 μm?

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Unlike erythrocytes that have a very rigid shape and almost cannot change their size (hence the size distribution is indicative and can be used for diagnostic purposes in medicine), lymphocytes can change their size in a wider range, this is why you see the numbers 6-9 and 10-15 μm.

And they indeed cluster into several different groups: so-called "large granular lymphocytes", also known as NK-cells or "natural killers" (usually >10μm) and "small granular lymphocytes", constituted by a large family of T- and B-lymphocites(usually <10μm). But this clustering is not really distinct, for as I said above, both NK and T/B-cells can shrink (if the osmolarity of the external medium grows due to acidosis, inflammation etc.) and swell (on binding many IgE/IgG complexes, on certain cell factor released etc.). Besides, there are also some intermediate size cells, called NKT-cells, that also flatten the distribution.

So, what you can definitely say is that the typical size distribution of lymphocytes has two peaks: around 8 and 12 μm respectively.

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