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According to many sources including wikipedia, there are haematopoietic stem cell derived dendritic cells in the blood.

haematopoietic cell lines

  • figure 1 - haematopoietic cell lines - ref

Despite of this, when I examine a blood smear, I cannot find dendritic cells and I did not find any blood smear example on the internet, which depicts dendritic cells.

blood smear

  • figure 2 - blood smear cell lines - ref

Why is that?

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    $\begingroup$ I've identified both plasmacytoid and myeloid DCs (albeit at very low numbers) in PBMCs derived from whole blood of (presumably healthy) volunteers by flow, so they are there. I don't recall off the top of my head the exact numbers, but it was far less than 1% of PBMCs (dozens to hundreds of counts in a 1-2 million cell sample, IIRC), meaning an incredibly small proportion of overall cells in the blood. Therefore, the chances of seeing and correctly identifying them in a smear are rather low. $\endgroup$ – MattDMo Apr 24 '16 at 14:59
  • $\begingroup$ @MattDMo Does it look like this? google.hu/… It is extremely hard to find an image about them. In theory this is an activated mature DC moving from the skin to the lymph nodes. It appears to present antigens in the blood too. Actually I don't have deep knowledge about them right now, I just want to show an image to somebody. $\endgroup$ – inf3rno Apr 24 '16 at 15:46
  • $\begingroup$ @MattDMo According to this sciencedirect.com/science/article/pii/S0022175916300217?np=y normally about 0.5% of the WBC-s are some kind of DCs. Interesting. :D I probably confuse them with monocytes, and the image I linked is probably fake. Not sure yet. $\endgroup$ – inf3rno Apr 24 '16 at 16:33
  • $\begingroup$ This was by regular flow cytometry, so I wasn't taking images of them. All the images I've seen of DC in vivo are along the lines of these - the staining is somewhat diffuse because of all the ruffles and extensions. If you do a search for "mature dendritic cell SEM" you'll find a bunch of scanning electron microscopy images of the cells in 3D. $\endgroup$ – MattDMo Apr 24 '16 at 17:48
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The dendritic cells are antigen presenting cells that process and present pathogens to the T Cells.

As such they are present in interfaces where foreign organisms are frequently present. Eg: Skin.

When these cells get activated i.e. come in contact with pathogens, they process them and move to lymph nodes to present the processed antigen.

Thus these immune cells do not circulate in blood once they reach the target tissue but reside in the tissues and found in lymph nodes on activation.

Further, from Kuby's Immunology:

Dendritic cells can be difficult to isolate because the conventional procedures for cell isolation tend to damage their long extensions. The development of isolation techniques that employ enzymes and gentler dispersion has facilitated isolation of these cells for study in vitro. - Kuby Immunology 5Ed. P.42

Since blood is the most accessible tissue for clinical studies, we set out to extend the findings that were reported in mouse blood to humans. However, when we tried to induce DC growth by adding GM-CSF to human blood, we identified actively proliferating DC aggregates only infrequently - Proliferating Dendritic Cell Progenitors in Human Blood By Nikolaus Romani, Stefan Gruner, et.al.

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  • $\begingroup$ I need some references about the "do not circulate in blood" part. ncbi.nlm.nih.gov/pmc/articles/PMC3809702/figure/fig02 $\endgroup$ – inf3rno Apr 24 '16 at 12:21
  • $\begingroup$ "conventional procedures for cell isolation tend to damage their long extensions" - I think this is about flow cytometry. I don't think that blood smear creation would damage their long extensions. E.g. sickle cells and reactive lymphocytes are not damaged by this procedure. According to my previous link DC-s in blood do not have such long extensions, they are not mature enough... I believe they can easily confused with other cells, and some of them are derived from monocytes. $\endgroup$ – inf3rno Apr 24 '16 at 12:27

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