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Do macrophages in the dermis consume any foreign invader to the body, or simply pathogens and cellular debris? For example, would macrophages consume vitamins or medications injected into the dermal layer of the skin? Thank you

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Macrophages and dendritic cells generally recognize large foreign complexes. They often do this through the recognition of pathogen or damage-associated molecular patterns (PAMPs or DAMPs). Otherwise, cell-mediated pathways involve cyto/chemokines which direct macrophages and the like in the region. We also have to consider, does the compound in question belong in the region, or is it insoluble amongst the tissue? In these cases, the compounds are also degraded/engulfed. So how does the body go about recognizing small, soluble foreign molecules?

These are called haptens. If these small molecules can bind to a larger carrier complex in your body, so that it now looks like there's something foreign, regardless if the initial carrier complex was foreign or not, this may present cause for immunologic reponse (as in the mechanism of urushiol, the toxin in poison ivy). There are certain T-cells, like NKT and γδ T cells, which can "see" these antigens despite their lacking a type I or II MHC presentation.

The short answer is it depends on what the injected drug does once it begins interacting with your physiology. Vitamins, on the other hand, are regularly uptaken by cells for metabolism, and I wouldn't imagine macrophages engulfing exogenous vitamins outside of it's needs on their own.

Source: Cellular and Molecular Immunology, 8th ED.

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  • $\begingroup$ "... or is it insoluble amongst the tissue..." Would you say, current covid vaccine "nano lipid particles" are insoluble in that sense? By any chance, are your familiar with "lipid particles"? And, please correct: I do think, with "lipids", that they "they turn the other cheek", that they have a non-polar coating, outside. I have heard, by the way, that lipids may become polarized, attracting proteins, in the blood. $\endgroup$ – Peter Bernhard Dec 17 '20 at 17:11
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Macrophages take up pathogens to present them to other specific immune cells. The things they take up via phagocytosis have specific markers on them called PAMPs (Pathogen Associated Molecular Patterns). Hence they won't phagocytose locally injected substances (until and unless it has a PAMP like region).

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  • $\begingroup$ This is utmost interesting to me (so long ago you answered) in the framework of Corona crisis and the newly introduced RNA-vaccines. These come in "Nano Lipids". Would you say they are being uptaken by phagocyt.? ( their antigens must be presented by APC for vaccination to be effective). Infering from your answer: Nano lipd particels wouldn't be "deliberately" engulfed and taken up (I like your answer as it alludes to some relationship between receptor uptake and presentation) - nanos would be uptaken through diffusion - not different from any other cell that is no APC? $\endgroup$ – Peter Bernhard Dec 17 '20 at 16:55

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