How exactly does chemotaxis work?

[Dawgy]

TL;DR

I'm looking for an in depth understanding of chemotaxis

I recently saw a video online of a white blood cell taking out a bacterium. This engaged my curiosity and I wanted to know why the WBC and bacterium behave the way they do. I learned about chemotaxis, but none of the explanations I have found satisfy my curiosity. What exactly is happening between the receptors and attractants/repellents? Why does the attractant or repellent cause the receptor to behave in a certain way?

 

[jim mcnamara]

I want to give you an answer you can understand. What you saw was an immune response in action.

Anyone reading this who knows inflammation and immune responses will agree this is not the most accurate explanation available. But you will get the idea in spite of the anthropomorphized answer.

The complexity of that operation is pretty large. It is part of inflammation. Which is usually an ongoing process, that for simplicity, has cells bursting and/or leaving behind attractant chemicals for some other buddies that act like vacuum cleaners. I think you were watching one of the cleaner cells ingest another cell that probably had been painted with an "eat me" chemical signal,

Chemotaxis means movement of a cell toward (or away from) a source of a chemical. Cells in an aqueous environment usually swim with flagella. This is one way they can move. There are others. The biochemistry of movement is not trivial. And not all suited for a for forums like PF.

At one end of the reference book spectrum there is a science comic:
Science Comics: Plagues: The Microscopic Battlefield
Part of: Science Comics (20 Books) | by Falynn Koch | Aug 29, 2017

A little further up the food chain:
How the Immune System Works (The How it Works Series)
Part of: The How it Works Series (4 Books) | by Lauren M. Sompayrac | Apr 15, 2019

College, a bit dated
The Immune System, 4th Edition
by Peter Parham | Oct 1, 2014

NIH has immunology study section for scientists and physicians:

Google for "nih immunology study sections" - there are lots of them

For world class animations showing movement based on biochemical processes at the protein level see: https://www.microbe.tv/twiv/twiv-726/

 

[BillTre]

At first I resisted answering this question because it is as @jim mcnamara said it is very complex, especially in eukaryotes (the case with the immune system).
Chemotaxis is a form of behavior. Moving upgradient in a chemical concentration It can take different forms in different organisms.

Bacterial chemotaxis is much simpler that that involved in the immune response. Mutations affecting the process have been collected in bacteria which have helped to reveal how it works.
Bacteria can move in a direction or tumble and move forward in some other (random-ish) direction. If they are moving downgradient (in an undesired direction) it will tumble more frequently, until it starts going upgradient, in which case it tumbles less frequently.
The mutations have identified the genes involved. The proteins identified are involved in flagella function (bacterial flagella are quite different from eukaryotic flagella), the sensing of the chemical concentration, and the signalling from the sensors to the flagella.

This is not how it works with the much more complex immune cells, but it is chemotaxis.

Here is a wikipedia chemotaxis article that covers both bacterial and eukaryotic chemotaxis.