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Oh medtwitter I have found one of my coolest evolutionary facts ever! I was looking at evolution of innate immune system. It's very old, below vertebrates. We didn't get adaptive immunity until fish.
Relies on pattern recognition receptors, pentraxins (like CRP), toll like receptors, FREPS (fibrinogen related proteins) and generally revolve around agglutination, clotting and phagocytosis.
Anyway insects have another thing they do as an innate immune response. Melanisation. As in yes melanin. All melanin is created from catechol precursors like tyrosine/dopa (similar to melatonin also!) but they go through an oxidised quinone phase
That are very toxic and oxidise proteins/lipids they touch. In human melanocytes melanin therefore is produced in melanosomes to keep it compartmentalised.
However if you secrete the precursors around a pathogen it becomes sealed by a layer of melanin that inside contains those highly oxidised products, and makes it easy to phagocytose all the remnants.
I mean I thought that was quite neat and at first glance it doesn't seem to happen/have immune roles outside insects. But then!!!! I remembered how many autoimmune diseases manifest as skin pigment defects and I wasn't convinced there isn't something in it.
So guess what! Turns out melanin is actually fungicidal at concentrations produced by melanocytes (the skin brigade!). Is this why fungal infection can cause weird pigmentations. Could we even use melanin to help us with this clinically?
Worth also noting that there are plenty of reports about melanised fungal infections in human brain tissue - looks like it is a defence against cryptococcus and c albicans.
Moreover appreciation that melanin is catechol derived explains why Addison's (adrenal), neurofibromatosis (CNS/cafe au lait)&other conditions are linked (it never made sense to me melanocytes should be neuroendocrine derived, but a catechol origin is sympathetic nervous system!)
Furthermore this realisation that dopamine is a highly oxidisable compound made me appreciate how vulnerable dopamine pathways in the brain are to oxidation (eg age related or sepsis related) and why we see changes in midbrain/substantia Nigra and forebrain.
Neuromelanin is an end oxidation product of dopamine found in the substantia nigra that helps protect the cell from further oxidative stress, like skin melanin.
However rather interesting is that the first thing dopamine oxidises to is aminochrome, a key feature of which is ability to join up with complex I and III of mitochondria thus cause energy deficit. Ergo delirium. Mitochondrial function known cause.
So there you go, plenty to learn from evolution. https://www.hindawi.com/journals/pd/2012/920953/
