define neutropenia

Last night I took Pads to radiotherapy. On the way back he was asking if I knew what 'neutropenic' meant. I said it was when your white blood cells are rather low, making you vulnerable to infections so that even a cold can be bad news. I didnt expect him to roll up, puppy-dog eyes included, to say that the medical staff had just told him he was neutropenic... I was thinking that would come with the heavier doses of chemo at New Year...