De-sensitisation is a good treatment when an allergen is difficult to avoid, such as grass pollen. It is recommended when allergy medications don't help or cause side effects, or when people prefer to avoid medications.

De-sensitisation is not a quick fix. It involves three to five years of treatment, resulting in five to ten years of reduced symptoms. After this symptoms may return. 

Allergen immunotherapy is given by regular injections in to the skin, or tablets under the tongue (sublingual). It is the closest thing we have to a cure for allergy.