I wonder if you misheard the doctor and they actually said T cells, which belong to the lymphocyte family (you know how when they run a differential
white blood cell count, there are 5 kinds they check?) T cells are key to immunity. It is possible for them to become malignant, which it looks like online is called T cell lymphoma. But if you have been over your cold awhile, if you get rechecked, you may be blessed to find out they are normal, if that's what he was talking about.