Hi Roz-
Cells are so cool! I'm a big bio nerd- my degree is in neuroscience and I just love this stuff. The body is amazing because cells are able to work in such intricate tandem, and for so long no one knew how they did it. Every single cell has the ability to communicate with its neighbors about its specific interior and exterior environment. And it does this by sending out literally thousands of different protein signals. Antibodies are just one type of protein signal to other cells. The G proteins that some of the articles posted mentioned indicate a change of metabolic state inside the cell and start a cascade of processes that change what's going on within the cell. We have an alphabet of 26 letters that we can arrange into thousands of words to communicate. Our body uses a similar system. It has thousands of signaling proteins. But what is it that turns these on? Often it is the conditions within the body, but just as often genes turn these signaling molecules on and off. Or at least the body's ability to produce them.

Hope this helps a little . . . it can get really complicated, but really this is really what it boils down to.
Linnie