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There's this mildly viral video which reportedly shows a white blood cell hunting a bacterium:

https://www.youtube.com/watch?v=JnlULOjUhSQ

What I don't understand is - at this scale, how does the white blood cell "know" to chase the bacterium and where to go? How does the bacterium "know" to run? Are they constantly shedding some bacterium-specific molecules that the white blood cell reacts to? Wouldn't the bacterium soon run out of molecules that way? Is there some other mechanism? What's going on here?

Note - I'm a total newbie; my own biology/chemistry knowledge is high-school level at best (and even then I graduated 16 years ago), so please try to explain it simply. :) I'm OK with looking up unknown terms on Wikipedia, but I probably won't be able to comprehend too much jargon. Thanks! :)

David
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Vilx-
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    Sadly if you want a scientific answer, the question is too broad. We would have to delve into motility, immune and non-immune signaling, membrane biology of pathogens and commensals, the repertoire of immune cells, and the entirety of many kinds of response. For a rudimentary answer, try finding inspiration here, an article in Scientific American from 1998, and here on proteins that work as scouts. – S Pr Nov 26 '19 at 14:25
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    @SPr - So... the gist that I get from these three links is that the invading cells do shed material all the time, and it's this material which leaves a "trail" for the white blood cell to follow. The invading cell in the video is probably not "running away" but rather moving randomly, and perhaps being pushed a bit (via fluid dynamics) by the following white blood cell. – Vilx- Nov 26 '19 at 15:05
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    @SPr - The exact details of losing the material and detecting it are, of course, complicated (and it seems there are many variations when it comes to the technical details), but that's the general principle. – Vilx- Nov 26 '19 at 15:06
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    Water (or viscoelastic fluids like blood) is not static; water molecules are constantly moving or vibrating. That poor little diplococcus isn't swimming away from the white blood cell; it's simply being moved by kinetic forces. The bacteria in this video are being pushed by the fluid in front of the moving phagocyte, making it appear to be 'running away'. The WBC is capable of movement and what you're seeing is chemotaxis. While it's more complicated than this, no, the bacterium won't run out of whatever it is that the WBC is responding to. – anongoodnurse Nov 26 '19 at 16:28
  • @anongoodnurse - That poor little diplococcus isn't swimming away from the white blood cell; it's simply being moved by kinetic forces. Yes, I suspected as much. – Vilx- Nov 26 '19 at 17:04

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