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sharplydressedman t1_iuicqhm wrote

At a very simple level, it's all concentration gradients. Kind of like a dog can sniff and follow a scent trail from a very faint signature, following the trail until it is eventually on top of the target. Immune cells like the one in the video (presumably a monocyte or macrophage) have receptors specifically designed for Pathogen-Associated-Molecular-Patterns (PAMPs). PAMPs are molecules that are found on bacteria, fungi, viruses etc that our immune cells have evolved to recognize with receptors specific to them (antibodies not needed). An example of a PAMP is endotoxin/LPS that is a part of the bacterial cell wall. So for example, the bacteria sheds LPS or other pieces of its cell wall as it floats around, and the immune cell "sniffs" it out with its receptor and starts following the trail.

It gets more complex. There are hierarchical signals for what determines which direction an immune cell will migrate. For example, if local tissue cells realize there is an active ongoing infection, they will secrete "red flag" signals to recruit nearby immune cells to the area. These signals are called chemokines. So the immune cell floating around your blood will first detect the chemokines and realize something is wrong, and will enter the area where they are coming from. From that point on, if it senses PAMPs (the bacterial molecules), it'll switch and start moving toward the bacteria.

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UpsetRabbinator t1_iuikr9c wrote

Has any bacteria evolved to leave false trail behind to confuse our WBc?

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dabman t1_iuim5ha wrote

Seems like this would be difficult. I suppose if a cell released a sudden massive amount of chemical trail, and then was able to taper it off, it could give the illusion that the white cell overshot its target and it should stop or reverse to find it. I imagine the immune system itself has evolved to target a chemical release trail that is more passively released though, and not something the bacterial cell could easily develop an ability to control. Many chemicals released are dissolved gases for example, and may readily diffuse through cell walls without much ability to be concentrated/controlled.

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[deleted] t1_iujvg8s wrote

[deleted]

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InnkaFriz t1_iujchnc wrote

Thanks for the info! Follow up - is there a “red list” of bacteria patterns? Because afaik we do have a lot of bacteria we basically rely on for our health, and I’d assume that there is some mechanism to ignore them

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sharplydressedman t1_iujzkmj wrote

Well, yes and no - this will require a bit of a deeper dive into immunology. Our immune system has two general branches, the adaptive (slower but can "learn") and innate (quick but limited to pre-determined common patterns). There isn't a "red list" per se for the innate immune system since it is evolutionarily more efficient for our innate immune cells to have the receptors for the definitely dangerous patterns, and let the adaptive immune system "learn" which patterns are safe.

As a metaphor for the innate immune system, the TSA displays a list of things that are definitely banned on planes. They may not need to have a list of things that are "definitely safe", they can figure that out along the way.

Anyway to return to your question, the adaptive immune system DOES have the ability to identify molecules that are safe. "Mucosal tolerance" refers to the ability of the body to suppress immune responses against antigens that are encountered in the gastrointestinal tract. This is not only to protect the commensal bacteria that live in our intestines, but also prevents our immune system from flaring up against the molecules in our food.

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CynicalDarkFox t1_iujieo7 wrote

From what I know, that helpful bacteria is more situated in the GI tract rather than floating around the body.

However, it wouldn’t be a long shot to assume that helpful/neutral bacteria wouldn’t give off these chemicals that a pathogen would either since they aren’t exactly there to propagate selfishly.

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WasabiSteak t1_iujm7cp wrote

Maybe they're not "selfish" in the GI tract, but I don't think they would behave once they get into the bloodstream.

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Jimmy_Smith t1_iujz6c0 wrote

These are called opportunistic pathogens. They may normal gut bacteria like e. coli, but when it enters the bloodstream or travels up your urethra to your kidneys they will kill you.

Now some of these bacteria may stay put either because they are inhibited by competing bacteria which is why some research focuses on transplating gut microbiome, but sometimes bacteria wait until they have enough around for a massive attack through quorum sensing.

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tedivm t1_iuk9uu7 wrote

>but sometimes bacteria wait until they have enough around for a massive attack through quorum sensing.

How does that work?

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sporesatemygoldfish t1_iujr177 wrote

That is fascinating! Thank you for the understandable response.

It made me smile to think of battles that go on inside my body. WHITE KNIGHTS!

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lupadim t1_iuk3zcf wrote

I can't grasp how this overly complex system can come to exist without intelligent design

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WordsNumbersAndStats t1_iukavvb wrote

It is essentially trial and error repeated hundreds of millions of times over hundreds of millions of years.

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keloidoscope t1_iuk7rx5 wrote

if you think it's overly complex, how does that argue for intelligent design?

Natural selection only rewards economy and elegance when it confers a selective advantage, and inherited traits don't get magically optimised even when they start to get baroque or unwieldy in descendant species.

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