People prone to allergic reactions and are sensitive to certain types of material should always check the types of duvet covers. If you are sensitive to certain dyes, fabrics or materials, the best kind of duvet for you is the one that has been recommended to you by your doctor.
A duvet with a high cotton content is usually the best type for those who suffer from allergies, as these are often made from the most natural synthetics. Bed linen woven quite tightly is more resistant to dust mites and bacteria, so a duvet cover with a high thread count may be what you need.
Many people who suffer from allergies will often want to wash their bed linen every 3-4 weeks, so check that your cover can withstand this (most duvet covers are machine washable and can withstand many washes).