What can you tell me about the success rates of desensitization?

Please clarify. From what i know of desensitization, it is a form of therapy for allergies that is supposed to make people less sensitive to things they are allergic to, like chocolate. They are exposed to a small amount, which gives a reaction, which, over time, gets less. It has been done for a long time with decent results, but is not employed as much as it once was.