How can a person be desensitized to an allergen?

Immunotherapy. The simple answer is allergy immunotherapy. This is typically done for airborne allergens or bees and is the process by which giving the patient back increasing doses of the allergen(s) they will have less and less symptoms. Allergy vaccine therapy is not a cure but can induce long term modifications in the immune system so that the targeted allergens clinically cause less problems.
Immunotherapy. Immunotherapy involves giving an allergic person, small doses of what they are allergic to. The dose is slowly increased so that the individual becomes desensitized. Desensitization can be done by subcutaneous injections or by a newly developed program of sublingual drops.