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Overly simplified, macrophages recognise pathogenic patterns and endocytose anything that matches them.

That also works on bacteria, which are quite often very mobile. What if a bacterium was just randomly twitching around when a macrophage recognised it - would it be possible for it to "swim away" (as a reaction to the macrophage or just randomly)? Or do macrophages employ some sort of fixation mechanism to keep anything they recognise close until it is engulfed? Or is engulfment in itself just so quick?

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up vote 2 down vote accepted

As an alternative explanation I refer to this video.

Crawling Neutrophil Chasing a Bacterium

In addition to a high affinity between the target bacterium and the macrophage, sensing the bacterium will also trigger amoeboid movement to chase down a pathogen.

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Do you know how it senses the bacterium? edit: Might put this as a separate question actually... – Armatus Jun 18 '12 at 13:33

The explanation one of my tutors offered was this:

In cell recognition, it is very common for interactions between proteins to be based on weak interactions, but a certain degree of binding can trigger a (conformational) change which will instead establish a very strong interaction between the proteins involved.

In other words, if the contact is specific enough to 'count as a recognised pathogen', the interactions will immediately change to strong forces and the pathogen will not be able to move away.

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