Many people think they are allergic to down. In almost all cases this is not the case.
The real cause
Whoever wakes in the morning with irritated eyes or a blocked nose, in nine cases out of ten is reacting not to the filling of the duvet, but to dust mites. Dust mites are microscopic. They feed on skin flakes and favour a warm, moist habitat. A bed is precisely that, not your bed in particular, every bed.
What a good duvet does
A good down duvet is not an attractive place for dust mites. The tight weave of a good casing keeps them out. And a professional wash, every five to seven years, dispels what might settle on the surface.
Truly allergic
Whoever is truly allergic to down, and it does occur, recognises it through direct contact with an open cluster. Under a finished duvet they generally notice nothing. For those who want certainty, hypoallergenic variants exist with extra-tight casings, through which no cluster ever passes.
In practice, after nearly a century: most people who thought they were allergic to our duvets, slept softly under them after an adjustment in their routine.