Hi mj,
I came across a number of articles this evening while researching another question which discuss sensitivity. The constant theme is what you mention that repeated exposure creates greater weakness. There have been some caveats in those observations though. As a person ages, they have decreased sensitivity (generally). As a person has increased time between exposures, they may be less likely to be sensitive. And some people just turn out to be no longer allergic (without stated reason). So there is high variability here. What I have seen and read is pretty much that the reactions vary from person to person, time to time, and season to season.
Here are some links to references:
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