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