How exactly does an IgG antibody control in ChIP work? - (Oct/08/2011 )

Hi to everybody,
I will soon start chromatin immunoprecipitations and am just wondering what exactly an IgG control does tell me.
For example I am working with murine cells. So I will have to use a mouse IgG. But what exactly is this IgG antibody control? Is it a mixture of unspecific antibodies? How is it generated/produced?
I don't get the exact theory behind this. I did not find an answer in the internet.
So would be great, if someone could explain it to me.

It tells you that what you are precipitating is not something which just binds to IgG...

Ok, maybe I did not express myself very good: I understand what it tells me.
I just don't understand, what the IgG control exactly is. I mean, my antibody which I use for precipitation may als be of IgG type.
So what is this IgG control in a physiological context? Is it a mixture of IgG molecules isolated from the animal of which's cells I perform the ChIP? E.g. I buy a from the manufacturer as "normal mouse IgG" prescribed IgG control. Is this just the whole amount of IgGs isolated from mouse?

Basically yes, it is the normal complement of IgG that is floating around in the serum at any one time.