How to identify normal leukocytes in a blood smear

Q. I’m not sure if I can identify leukocytes correctly. Could you give me some tips? Thanks very much.

A. Sure! When you are just starting out in hematopathology, it can be a bit overwhelming. It’s really not as difficult as it seems. There are just 5 kinds of cells that you see in normal peripheral blood, and with a few guidelines, you can tell them apart pretty easily.

Neutrophils
These are the most numerous white cells in normal blood. There are two in the above image (from WebPath), one at about 3 o’clock and one at about 10 o’clock. They are part of a category of white cells called “granulocytes,” which refers to the cytoplasmic granules you see in these cells. In neutrophils, the cytoplasmic granules are mostly small, pale peachy-pink granules. These granules (called “specific granules”) are what give the neutrophil cytoplasm its pinkish color. There are also scattered larger, dark purple (or “azurophilic”) granules. These are called “primary granules” because they are the granules that appear first as the neutrophil matures. If you forget the thing about neutrophil maturation, you can remember which is which by remembering that the p words go together (primary=purple).

The nucleus of a normal neutrophil is also unique-looking. It’s segmented – pinched off into different sections, like sausage links – rather than round, like most other cells. Neutrophils are sometimes called polymorphonuclear leukocytes because there are several (poly) bodies (morpho) in the nucleus (nuclear). Rarely, you might see a “band” cell (which is the neutrophil at 10 o’clock), which is the stage of neutrophil right before the nucleus becomes segmented. Neutrophil chromatin in general is clumpy, and you can’t see any nucleoli.

Lymphocytes
These are the second most numerous type of white cell in normal blood. There’s one lymphocyte at 8 o’clock in the above image. Lymphocytes are generally a bit smaller than neutrophils, and the thing that sets them apart is their chromatin, which is both clumpy and smudgy at the same time. It looks like someone took a finger and rubbed the nucleus before the ink fully dried. Although there are clumps in lymphocyte chromatin, there aren’t discrete white spaces between the clumps, like you see in neutrophil chromatin. Check out the above image and you’ll see what I mean. Sometimes you’ll see lymphocytes that are a bit larger, with more cytoplasm and maybe a few coarse granules. T cells often have this appearance (though you really can’t tell for sure without doing some special studies).

Monocytes
Monocytes are big cells (there’s one at about 8 o’clock above) with lots of cytoplasm. The cytoplasm often has a “dishwater” appearance, meaning it is sort of cloudy and grayish. Sometimes, as in the cell above, it’s more of a pale purple color. You can see some fine purple granules scattered about as well. The nucleus is big and it’s usually indented, or horseshoe shaped. The chromatin is pretty fine (finer than neutrophil or lymphocyte chromatin), and it has a weird “raked” appearance on high power (it looks like someone messed up the chromatin by dragging a rake across it).

Eosinophils
These cells, along with basophils, are probably the easiest to spot (there’s an eosinophil at 2 o’clock above). Both eosinophils and basophils are granulocytes. The granules in eosinophils are beautiful – they are large, luminous, and reddish-orange. The word eosin comes from the Greek word eos, which means “flush of the dawn sky.” Very cool name for these gorgeous, sunrise-colored granules. The nucleus is nothing to write home about, really – it’s segmented into a few different parts, and it looks kind of like a neutrophil nucleus.

Basophils
You can tell a basophil from a mile away: it’s the cell with the big, super-dark-purple-blue granules (there’s one at 4 o’clock above). The granules are so numerous and dark that they often obscure the nucleus (which is a rather boring nucleus, usually divided into two segments). Basophils are the least numerous of all white blood cells – you may have to look several fields to find one.

And that’s it! When you start looking at different diseases (like infection, or leukemia), it gets a bit more complicated, because you often see immature cells out in the blood. But for now, you can just focus on the normal, mature white cells. Once you get familiar with these, they start looking like little friends that you happily recognize from across the street.