What's the difference between an allergy encasement and a regular mattress protector?

A protector usually covers only the top and sides like a fitted sheet and stops spills. An allergy or bed-bug encasement is six-sided with a zipper and uses a tightly woven fabric (typically a pore size around 10 microns or smaller) to physically block dust mite allergens and bed bugs. If dust-mite or asthma symptoms are the reason you're shopping, you need an encasement, not a protector.