Sometimes, you just can’t tell what kind of tumor you’re looking at under the microscope. In acute leukemia, for example, some cases have distinctive features (like Auer rods) that tell you what kind of leukemia it is – but other cases have no clues.

In cases where clues are minimal – particularly in hematolopoietic or lymphoid neoplasms – you can do flow cytometry to see what markers are on the surface of the cells. This is a test that uses fluorescent antibodies to tag molecules on the surface of cells. The flow cytometer, which is super fancy, has a teeny tube that allows the cells to flow one at a time past a laser beam (check out the diagram above). In addition to telling what kinds of markers a cell has (by whether it fluoresces with the antibodies you used), you can also sort cells by size and complexity. It’s an incredibly useful technique that’s used for lots of different purposes, one of the most common (in hospital practice, anyway) being to find out what markers are on the surface of cells. In a bland-looking leukemia case, for example, if you did flow cytometry and saw that the cells expressed CD 13 and CD 33, you’d know the cells were myeloid, and that it was most likely an acute myeloid leukemia.

The “CD” in the name of these markers, by the way, stands for “cluster designation.” It’s just a way of referring to the different molecules on the surface of cells so that instead of having all kinds of different names for these molecules, there is just one name (a number, actually) for each molecule. You know how it is: unless you have a single system, you’ll get all kinds of different names for the same thing.

It might be a good idea to know some of these markers. You already know a few: CD3, for example, is a CD marker that’s on the surface of all mature T cells, CD4 is on helper T cells and CD8 is on cytotoxic T cells. There are over 350 CD markers, so obviously you don’t have to know every single one. But some of them are used so commonly that it would probably benefit you to know what they are and how they are used.

Here is a list of commonly-used markers, sent in by a very nice reader who knows a lot about this stuff. Some markers are used in more than one instance (for example, CD15 is present on both Reed-Sternberg cells and neutrophils). Note that they are listed, for the most part, in numerical order. Note also that sometimes it’s the absence of a marker that helps you with the diagnosis. For example, if you have a lymphoid neoplasm in which the cells are small and mature looking, and by flow those cells are CD5 positive but CD23 negative, you’d be able to rule out chronic lymphocytic leukemia and lean towards a diagnosis of mantle cell lymphoma. Those small lymphoid neoplasms are very interesting by the way – and flow is super helpful with making a specific diagnosis.

And now, the list:

CD1a, CD207: Langerhan cell histiocytosis cells
CD2, CD3, CD4, CD5, CD7, CD8: T cells
CD10: Early pre-B cells (immature B cells)
CD11c, CD25, CD103, CD123: Hairy cell leukemia cells
CD13, CD33, CD117: Myeloid cells
CD14, CD64: Monocytic cells (positive in AML-M4 and AML-M5)
CD15: Reed-Sternberg cells, neutrophils
CD16, CD56: Natural killer cells
CD19, CD20, CD21, CD22 : B cells
CD23 and CD5 : Chronic lymphocytic leukemia/small lymphocytic lymphoma
CD23 negative and CD5 positive: Mantle cell lymphoma cells
CD30 and CD15: Reed-Sternberg cells
CD30 positive and CD15 negative: Anaplastic large cell lymphoma cells
CD31: Endothelial cells (positive in angiosarcoma)
CD33: Myeloid cells and precursors
CD34: Stem cells (also positive in angiosarcoma)
CD41, CD61: Megakaryocytes and platelets (positive in AML-M7)
CD45 : All leukocytes (except Reed-Sternberg cells!)
CD45 RO: Memory T cells
CD45 RA: Naive T cells
CD68: Histiocytes (positive in malignant fibrous histiocytosis)
CD99: Ewings sarcoma cells
CD117: Gastrointestinal stromal tumor (GIST) cells, mast cells (positive in mastocytosis), myeloid cells