The most common allergen is the dust mite! Dust mites are tiny creatures of the spider family that feed of your old, discarded flakes of skin. They and their droppings, in particular, are highly allergenic. You can reduce the number of these mites that infest your mattress by using hypoallergenic mattress covers or encasements. Some mattress manufacturers use covers that have been treated to eradicate dust mites. Check the label and other information that comes with your mattress for any information on allergies. If you suffer from an allergy, then you are advised to use a hypoallergenic mattress topper or pad. You can find information on hypoallergenic toppers and pads here.