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Monocytes are phagocytes that "evolve" or "differentiate" into macrophages. I read that there are monocytes in the blood stream and vessels and that macrophages are found mainly in other tissues and in the lymphatic system.

My question: are there macrophages (and not merely monocytes) in the blood vessels and within the blood stream? And if there are, are they numerous there or sparse?

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Usually, monocytes and neutrophils are present(there are other cells like Lymphocytes and other leukocytes), the only time a macrophage will be present is when inflammation occurs, I suspect this is due to the large size of the macrophage, it would be difficult for it to travel in the bloodstream, as it is quite big, compared to the small size of a monocyte. During inflammation, vascular dilation and permeability increase, which makes it easier for cells and proteins to diffuse across the endothelium.

A source that macrophages are present in all tissues: https://www.researchgate.net/publication/227897252_Macrophages_in_the_embryo_and_beyond_Much_more_than_just_giant_phagocytes

A source that talks about all the leukocytes and lymphocytes present in the blood normally: http://www.histology.leeds.ac.uk/blood/blood_wbc.php

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    $\begingroup$ Welcome to Biology.SE! This looks like a good answer, but answers are much more likely to receive a favorable response if you include supporting references (primary literature is best). Without that support, your answer is indistinguishable from opinion. This is a good example of how to format references. ——— You may also want to take the tour and then consult the help pages for additional advice on How to Answer effectively on this site. Thank you! 😊 $\endgroup$
    – tyersome
    Commented May 5, 2020 at 5:41
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Macrophages are white blood cells and I’m pretty sure that leukocytes are present in blood that flows throughout the body and hence they are found in tissues. Typically, more number of macrophages are present in the blood when there is some form of invasion by bacteria or virus.

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    $\begingroup$ This answer is less specific than the question (leukocytes vs. macrophages). $\endgroup$
    – R Stephan
    Commented Jul 11, 2018 at 5:43

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