I wonder if anyone knows a good hack for this problem. I suspect physical exercise works to some extent. Perhaps meditation?
When I don't, it's usually because the people I'm meeting with just are unwell and germy, even if they aren't actively coughing and sniffling. In such cases, showering as promptly as possible afterwards and engaging in self care to support my immune system helps me recover faster.
I will note the coffee and alcohol mentioned in another comment both have moderate medicinal effects, as does spicy food. (But it does so at a cost because it's a stimulant, not nutritional support. It doesn't give the adrenals more to work with. It just runs them on high, basically.)
Alcohol is an antiseptic and helps kill germs. Anecdotally, I have heard of cases where an improperly prepared meal caused food poisoning in the folks who did not have alcohol with the meal, but not in those who did have alcohol with the meal.
Caffeine boosts the adrenals, which helps with immune function generally and allergies in specific.
YMMV and similar disclaimers.
The best I figured out is to maximize energy levels at all times - increases likelihood you'll have any to spare. (Diet, excercise, meditation and the hardest, good amount of sleep.) It is a limited improvement at best.