There is value in using Prednisone to bring the initial allergic symptoms under control. But it's not something that should be prescribed for more than a couple of weeks. There's too much of a chance it will induce symptoms of Cushings Disease long term.
If available OTC in Brazil then an anti-histamine such as Benedryl (1-2 mg/lb) should be tried. Check with the Vet on dosage. Or the Vet may prescribe another anti-histamine. If the skin is irritated then a short course of Antibiotics such as Cephalexin may be needed. It's not uncommon for skin allergies to have skin infections follow due to the scratching. If the combo of an anti-histamine and Antibiotic doesn't reduce the inflamation and itching, then it's time for a referral to a doggy allergy specialist to rule out other problems such as auto-immune disease, or to try to determine which allergen in the environment is causing the problem.
Assuming you can get the itching under control then after the fact, supplementing the diet with fish oil (mild anti-inflamatory properties due to the Omega-3 fatty acids), and Vitamin-E (dry succinate salt form for best absorption) will help repair the skin. The latest rage in the scientific literature is "Phellinus linteus" which is a Chinese mushroom preparation. As with all such herbs, potency and how its prepared might differ greatly. Buyer beware.