How to identify lymphocytes in a blood smear

Q. Currently I am in a residency course to finish up my training as a medical laboratory technician; for the next two weeks I’ll be doing nothing but cell differentials in the hematology lab. Today as I was skimming the abnormal slides I found that I was having some difficulty distinguishing lymphocytes (particularly plasmacytic lymphs) from plasma cells found in the peripheral blood. Any pointers? In addition, I’m having a similar issue making the distinction from activated lymphocytes and monocytes. Pesky lymphs…

A. Those are very legitimate questions and ones that trouble even people with lots of experience from time to time. The key to both of these problems (and most problems where you’re trying to distinguish one cell from another) is to look at the chromatin.

1. Lymphocytes vs. plasma cells vs. plasmacytoid lymphocytes

Lymphocyte chromatin has a unique look in that it is clumpy and smudgy at the same time. Check out the top photo of normal lymphs – there are light and dark areas (clumping) within the chromatin, but the distinction between the two is not sharp (it’s smudgy). It’s like you licked your thumb and smudged the chromatin. Okay, that’s a weird analogy, but whatever. Plasma cell chromatin is blocky and discrete; it is sometimes arranged in a “clock-face” pattern around the edge of the nucleus. Not smudgy. Plasmacytoid lymphs have the chromatin of a lymphocyte (clumpy and smudgy) but the cytoplasm of a plasma cell (eccentric nucleus with a clearing where the golgi apparatus is).

2. Reactive (activated) lymphocytes vs. monocytes

Reactive lymphocytes – particularly big ones – can look a lot like monocytes. Again, the key is to look at the chromatin. Large reactive lymphocytes are usually immunoblasts, and as such, they have a big nucleolus (or two). In the bottom photo, there is a big reactive lymphocyte (called a Downey 3 cell) on the right. These cells also have fine chromatin (it has to be fine, or you wouldn’t be seeing the nucleolus). Monocyte chromatin is more dense (no nucleoli) and has a “raked” appearance. It is like you dragged a tiny garden rake across the nucleus. Also, the nucleus is often kidney-bean or horse-shoe shaped, or at least has a nice indentation or two. In addition to the chromatin differences, there are cytoplasmic differences (though these are less consistent): monocyte cytoplasm is typically dishwater grey with tiny dust-like granules, whereas reactive lymphocyte cytoplasm is usually light blue (either pale light blue or a relatively bright light blue) and if granules are present, they tend to be larger.

It just takes time and practice. Show everything you’re wondering about to someone who’s been in the lab a while – that’s the best way to learn. Most techs – as you no doubt know – are really nice and very knowledgeable!