You can't take pseudoephedrine every day. Nor is pseudoephedrine used for allergies - it is a decongestant, not an antihistamine. Nor does immunotherapy "solve" allergies.
A simple google search before posting would have made your fictitious scenario a little bit more believable.
Guess what happens when allergens irritate the inside of your nose? You get congested.
> Nor is pseudoephedrine used for allergies
Technically true but it is often paired with loratadine in, e.g., Claritin-D. Many allergy sufferers also get congested sinuses.