Summary

And that's it - that's inflammation. So, to summarise:

Cells, such as macrophages and dendritic cells, recognise generic pathogen molecules called PAMPs, and molecules released by damaged cells (perhaps as the result of an infection) called DAMPs. This is achieved by the PAMPs and DAMPs binding with receptors on these cells called PRRs. When these cells recognise pathogens, they release cell derived inflammatory mediators, which initiate and/or take part in the inflammatory response. Alongside these mediators, we also have plasma derived mediators. These work in combination with the cell derived mediators. The systems which the plasma derived mediators are part of are not activated by cells. We will discuss the processes involved in the next subsection, but, suffice it to say that it is largely done by the mediators themselves, or other molecules present in the blood. These mediators recruit immunocompetent cells which, hopefully, remove the pathogen (or whatever initiated the response); or serve to contain the pathogen. (Some also have a direct effect on the pathogen.) This is done, for the most part, by inducing vasodilation and increasing vascular permeability (which increases the presence of immunocompetent cells in the required area and allows them to pass out of the blood and into the interstitial fluid), or by inducing chemotaxis (which almost "summons" immunocompetent cells). These cells then get to work removing the pathogens and when they have done so, healing or repair can begin. Because these mediators are quickly degraded in the body, they do not survive for very long. Also, as part of the inflammatory response, molecules which inhibit certain aspects of the response are released. Therefore, to sustain the immune response, a sustained presence of a pathogen is required. Otherwise, it will "fizzle out" naturally. Where the response is sustained - because of a difficult to remove initiator, or because the response has been wrongly initiated (say by one's own cells), or because of a defect - chronic inflammation results, which can be harmful.