You always can. For some amounts of allergenics, you can try just once, but you will not accomplish the immunity because you will die first.

Yes. That’s the principle behind getting allergy shots. The doctor doesn’t actually give you medicine to counteract the allergy, he’s actually giving you gradually increasing doses of the material you’re allergic to. For example, ragweed, pollen, cat dandruff, etc.

Allergies are a tricky thing. Some people have no allergies for years and develop it out of the blue. Other people have allergies for years and then have it disappear forever.

It also depends on what you’re allergic to. If you’re allergic to ragweed, and ragweed fails to bloom one year, then you won’t have allergy symptoms.

Homeopaths sometimes recommend consuming local-produced raw honey in small amounts at first and increasing intake over time. Supposedly this will alleviate allergic reactions to local pollen over time.

actually i have only seen this in babies. In Germany they have places where they keep babies out side, way back when, and they are less likely to develope an allergy.
but i am not sure about adults

Yes but the immune system can cause the reaction and you should develop immunity only under competent medical supervision. I read (long ago) that many people do not catch poison ivy the first time they are exposed to the sap. But their immune system kicks in and at the next exposure over-reacts. Some people have life-threatening reactions to peanuts, seafood or bee stings, etc. and should never try to treat themselves.

Yes. a good example is bee stings. some people are deathly allergic. It is possible to build up an immunity to this reaction to the point were it hardly even hurts. Some people who undergo bee sting therapy get injected with the equivalent of 100′s of bee stings.