My layman's understanding is that the key to desensitization is that it has to be below the level of a certain kind of reaction.
Allergies occur because the immune system incorrectly associates a substance with a bad reaction, and so attacks it as it would a pathogen. The problem is that the allergen isn't a pathogen, and so the immune system can't actually kill it.
There's a treatment for pet/pollen/etc. allergies that works by injecting a very small quantity of the allergenic substance every week, slowly building up tolerance. The body learns that the small dose didn't cause problems, and slowly gets accustomed to higher and higher doses. If the doctor sets the dosage too high, the body has an allergic reaction and then that allergic reaction reinforces the immune system's determination that the substance is dangerous. The treatment response to this is to drastically reduce the dosage and try again.
If the sibling comment's assertation that oral exposure was desensitizing was correct, that wouldn't explain why some people develop food allergies later in life. (As one anecdotal example, my Wilderness First Responder instructor was slowly getting more and more allergic to mangos.)
Allergies occur because the immune system incorrectly associates a substance with a bad reaction, and so attacks it as it would a pathogen. The problem is that the allergen isn't a pathogen, and so the immune system can't actually kill it.
There's a treatment for pet/pollen/etc. allergies that works by injecting a very small quantity of the allergenic substance every week, slowly building up tolerance. The body learns that the small dose didn't cause problems, and slowly gets accustomed to higher and higher doses. If the doctor sets the dosage too high, the body has an allergic reaction and then that allergic reaction reinforces the immune system's determination that the substance is dangerous. The treatment response to this is to drastically reduce the dosage and try again.
If the sibling comment's assertation that oral exposure was desensitizing was correct, that wouldn't explain why some people develop food allergies later in life. (As one anecdotal example, my Wilderness First Responder instructor was slowly getting more and more allergic to mangos.)