Chihuahuas are not hypoallergenic but a good thing is that they don't shed a lot of hair like some other dogs do so you're less likely to suffer from allergies relating to the Chihuahua.
Still though you could be allergic to a Chihuahua and if so you may not be able to keep the Chihuahua.
So if you're allergic to dogs then you'd still be allergic to Chihuahuas even though they don't shed as much but if it's just plain allergies then you're less likely to suffer from allergies being around a Chihuahua.