Identical twins have 70% chance to share same allergic diseases vs 40% for non identical twins, and both of which is vastly more common than their prevalence in the general population. Though this also demonstrates environmental factors play a significant role.
That said, the human immune system includes significant randomness so environment may be even less important than generally assumed.
That said, environment is a factor. People in the developing world test positive for fewer though by no means zero allergies compared to their relatives in the developed world.
One thing I think people forget with this stuff, 50 years ago many people just weren’t exposed. I live in Ireland; when my grandparents were kids everyone in the country could’ve been allergic to sesame and no-one would have noticed; it just wasn’t part of the food culture.
Now, it is. In general, particularly in rural areas, most peoples’ diets were extremely restrictive until pretty recently; in industrialised countries people are exposed to a lot more variety than a couple of generations back.
The huge difference between identical and non identical twins is suggestive of a very strong genetic component.