Dust buildup is normal, especially if someone has pets. Because of your allergy you probably keep the area around your computer cleaner than most people!
In the land before time I did PC maintenance and sometimes you could remove dust from under a CPU fan in one single piece because it was matted together ... nasty, but very satisfying when you've finished cleaning and the computer just whirrs happily afterwards :-)
This brings back memories ... then I got a Thinkpad and just take it for granted that everything's fine, lol