“Hypoallergenic” means that the product or material is unlikely to trigger an allergic reaction. Hypoallergenic pillow covers comprise materials designed to prevent your allergies from flaring. Tightly woven fabrics or plastics discourage pet dander and dust from accumulating. They also prevent dust mites from burrowing inside pillows. Without these covers, mites will feed on the dust and fibres inside your pillows. Their excrement will trigger your allergies while you sleep.