Scientists scratch heads over why we itch

On a recent mountain trip, I was forced to scratch an itch on my leg during the night while half-asleep. In doing so, I inadvertently killed a tick which had chosen me for supper. Its dismembered corpse lodged under my fingernail and at that point I woke up. The extraction was automatic, performed in complete darkness and apparently beyond my conscious control at the time. It got me wondering. What if, at some point in our evolution, there had been a fatal disease that wiped out many of our early ancestors? Chances of survival were enhanced if the animal had an automatic and irresistible need to scratch. This urge is pretty wide-spread and in many large mammals leads to the beast rubbing its body hard against a tree or large stone. Without the insane need to do this, perhaps animals would be constantly covered in disease-spreading parasites. If this is the case - is it also possible that the other aspects of itching (such as those caused by allergic reactions) are simply annoying side-effects of the otherwise life-saving function?

I think that the act of scrathing an itch creates an instant localised pump for the blood and lymphatic system. Increasing the rate of flow of both to assist the immune system in getting rid of what caused the itch.
Either that or I smoked something interesting.

PaulStory is I think close to the explanation for itching/scratching. From an evolutionary POV, when insects or fungii or bacteria or thorns or anything else attaches itself to you, there is evolutionary benefit in developing a script for dislodging them. Scratching seems like a simple stereotyped behaviour that can handle a wide range of assaults. Like any other such behaviour of course, it can go wrong, resulting in compulsive scratching and ulceration; but like most evolved behaviours, there are pros and cons, and the pros outweigh the cons.

And as far as the sensation of itching, that'd be nothing more than an evolved signal to the brain that it's time to scratch.

Now, why is the Itchy and Scratchy theme song running through my head?