Another piece of anecdotal evidence. My wife has had allergy attacks constantly for years and years. Every morning she would wake up and have to blow her nose for a half hour. We started making fermented food (saurkraut, pickles, fermented garlic carrots) and at the same time buying/drinking keefir daily. This was just to be healthy and as a new hobby. However, within a week or two, her allergies subsided substantially. To the point where she would go for weeks at a time without having the morning nose blowing session. When she dials back the intake of fermented foods, allergies come back.

We haven't spent much time tracking it closely, but the relationship is obvious enough to be a no brainer for us.