HUH: Study: Thumb Sucking and Nail Biting in Children May Help Allergies. “The findings support the hygiene hypothesis, which says that early exposure to things like bacteria, viruses and allergens can prime the immune system to be stronger and better able to respond to any microbial attacks. It’s possible that the germs that children ingest from sucking their thumbs or biting their nails sets up certain populations of microbes in the gut that can educate the immune system and get it ready to mount attacks against other, more unfriendly and disease-causing germs.”