My mother is having cough from 3 years it settles down a bit and again becomes more. we have consulted doctors but nothing helped. all tests are normal. ige level seems to be increased. but no medicine helps. we can't find what she is allergic of?

Has she ever had a chest -x-ray? Does she smoke? Have your mother see an ENT specialist.

It is not feasible to provide a meaningful opinion without additional history, physical examination and may be some tests. Some medications, e.g., anti-hypertensive drugs can produce cough without any infection. It would be prudent to see a pulmonologist. Wish you good health!