Allergy from long period

Patient: M getting allergey from last 4 years..i dont know what is the reason is till now..alergic symptoms itchiness and skin changed to red colour..symptoms first start in the hair on the head after it will enter into the remaining using antihistamine tablets from 4 years..when i use the tablet, im having allergy again with in 2 days.. .. how can i solve my problem..Can u please suggest??