Why thankyou for bringing that to my attention. I did not know allergic rhinitis was associated with reduced risk.

Well, I'm going to pour out some thoughts. Different densities of eosinophils, also release of histamine, these have been noted altered in COVID19.

It could be so many things, such as production of more neutralising mucous, different viscosity.

Mast cell stimulation is involved in allergy and these two play a key role in antigen presentation and viral recognition.....

Neutrophils too are a key part of COVID.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5341103/#:~:text=Activated%20neutrophils%20may%20thus%20contribute,to%20a%20normally%20harmless%20antigen.

https://pubmed.ncbi.nlm.nih.gov/22918213/

Pendrin is also upregulated in asthma like allergic rhinitis and asthma seems also protective in COVID.

https://www.jimmunol.org/content/181/3/2203

It should induce antiviral effects by increasing the export of natural antiviral compounds including chloride ion and SCN.

https://www.atsjournals.org/doi/full/10.1165/rcmb.2018-0304OC

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3208616/

I would assume that the environment is already rather hostile to viruses. The existing inflamed neutrophils and the increased presence of other immune cells, the change in the thickness of the mucosal tissue in allergic rhinitis and the possible production of more antiviral mucous may be involved in a faster early response.

It might alter the chances of developing symptoms, it might alter chances of the virus establishing an infection. I would imagine it leads to a heavier early response and reduced permisiveness to infection, which protect against heavier infection.

Presumably the change in the nose is not just seen there but affects the upper and lower airway too.