Macrophage pathogen fixation
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.