Absolutely! My sons both have many allergies (severe environmental and anaphylactic food allergies), and when they have been exposed to one of their environmental allergens (i.e. cat) one of the first things we notice is a huge change in their behaviour, with other symptoms (hives, runny nose etc.) to follow. They will become really, really emotional and completely unreasonable, which makes a lot of sense considering how rotten allergies make you feel. Even after they are feeling better, they will continue to be more tired and irritable for several days afterwards or for the entire season of spring and most of Fall, when we can't avoid exposure to mold etc. This low energy/high irritability could be an after effect of their bodies being worn out from their immune system being constantly at battle, or side effects from the anti histamines or both.