Do you know what milk protein concentrate is? MPCs are created when milk is ultra-filtered through a process which drains out the lactose and keeps the milk proteins and other large molecules intact. And why should you care about that? Well, for one thing, since late last year, a lot of this foodstuff has been coming in from China, Ukraine, Poland and India, and has been ending up in processed cheese (go ahead, take a look at what's in that Kraft Single you're putting in your grilled cheese sandwich).
I don't think most of those countries have a good track record on processed food safety.
And the other bad thing is that by using MPCs, the price of real milk is affected and dairy farmers are taking it on the chin. So just say no to that easy melting processed cheese and enjoy the real thing.