User blog comment:Plexioth- xD/Monster Hunter 3 Ultimate/@comment-1758220-20130320013345/@comment-130804-20130320181414

Uh, no? Well, yes, they affect blights, but while monster attacks that are elemental in nature can be partially negated by defence, the elemental resistances help determine how much more or less your gear protects you against that element.

For example, lets say you have a set with 200 Def, with 20 water res and -8 dragon res. Lets assume the attacks hitting you are, say, 300 damage. Your gear's def makes that only deal 100. If it was a water attack, it gets reduced to only 80 damage because of the 20 water res. If it was a dragon, it would increase to 108 damage because of your negative dragon res.

Hope that helps