Where Do Blood Cells Come From?

 In the post on TNF inhibitors, we took a look at the white cells listed on a Complete Blood Count with differential.

Unfortunately, a discussion of white cells isn’t quite so straightforward.  There are actually a few different explanations of white cells and their behavior, and I suspect that if scientists really had all the answers, things would all fall in place and be much clearer than the current model.

All blood cells are formed from stem cells in the bone marrow.  It’s believed that the specific type of blood cell formed depends on which chemicals act on the stem cell.  By way of analogy, think about sperm & egg coming together – if the sperm contains a Y-chromosome, a boy develops; if the sperm contains an X-chromosome, a girl develops.  Same egg, but the end product varies depending on what it comes in contact with.  With that in mind, we return to cell development.  Stem cells that are acted on by interleukin-7 become lymphocytes (a kind of white cell); if they encounter a different chemical, they become a different type of cell.

I’ve found a number of similar theories as to which chemicals (cytokines & hormones) lead to the development of different types of cells.

Obviously the cell-development theory still needs work.  There’s a hole in the theory when natural killer cells are sometimes included and other times ignored (and then dendritic cells are tossed in as an afterthought, without any explanation).  It’s inelegant to sometimes group white cells 3-2 and other times group them 4-1 unless there’s a very good explanation of how those differences influence their behavior.  In other fields of study, when it’s not clear how something works (or if there are big exceptions to the theory), it means you haven’t yet figured out the right solution yet.  I believe this is true with what we know about cell development.

Be that as it may, a future post will investigate some of the cool things that white cells do, and how this knowledge influences RA treatment.

Edit to add:  be sure to read Former Scientist’s comment, below.

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IL = interleukin
EPO = erythropoietin
TPO = thrombopoietin
M-CSF = macrophage colony-stimulating factor
G-CSF = granulocyte colony-stimulating factor
GM-CSF = granulocyte-macrophage colony-stimulating factor