Similar to the "old friends" theory, I personally thought the issue with allergies and parasites suppressing allergies as a "calibration issue". Where evolution effectively set the dominant immune-systems to what would be hypersensitive in an environment without parasites order to compensate for parasites immuno-suppressing to protect themselves. Sort of like how sickle cell anemia genes were selected for its malaria resistance, in spite of how bad it is to be born with two copies of the gene.