I've anecdotally heard of a number of people with allergies who's allergic reactions are reduced by repeated exposure.

The most common refrain of those I've spoken with (primarily animal allergies), is that (e.g. for those with cat allergies) the allergic reaction was reduced (in some cases to nothing) for a specific cat after repeated exposure to said cat. The most interesting thing was that the effect only applied to encounters with that specific cat.