The allergy is to a carbohydrate produced by most mammals, with primates being a notable weird exception, but which also appears in some other things. One of them is apparently pretty ubiquitous in vegan packaged foods too, though I can't remember what it is. Also randomly shows up in basically any packaged food in non-disclosed ingredients. And if eating at restaurants, the threshold for what constitutes dangerous cross contamination with foods containing it is vastly lower than for typical allergies or other dietary restrictions so even a place following "proper" standards can still be dangerous.

So it's basically playing anaphylactic shock roulette if you eat anything other than poultry/eggs, fish, grain, fruit, and vegetables that you prepare from as close to basic ingredients as you can, although there's only one recorded death from it.