what makes common allergens common? Why doesn’t the population have a random but equally distributed allergy list?

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Why have I never heard of anyone with a broccoli allergy.

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This is not going to be a very satisfying answer, but…

So backing up, food allergies are caused by proteins, which are the building blocks of living things. The immune system works by recognising foreign proteins and learning to make an attack response. For example, the mRNA COVID vaccines program your body to make a protein which is present in the COVID virus. Your body learns to attack this protein, so that when the real virus shows up, the immune system attacks it.

Food allergies occur when the body thinks that a protein present in food is a foreign invader. The first time it is exposed to it, it prepares to attack it the next time. An allergic reaction consists of a massive immune over-reaction to this protein, such a large over-reaction that it is dangerous to the health of the person.

So why some proteins and not others? The answer to this is “we don’t know”. There are some things known to increase the chances. For example, if a protein is unusually stable in stomach acid, that increases the chances that it will be an allergen, which makes sense because that increases the exposure of the body to it. There are some other things which have been identified as risk factors, but none of them are universally present in all allergenic proteins. It’s likely that there is no neat satisfying answer and that there are a number of different very complicated answers. Biology tends to be like that. The answer to why tall people have tall children is “genes”, as much as 80% of variation in height is genes. But there is no “height gene”, in fact thousands of different genes influence height. In the same way, a protein being allergenic or not is probably a combination of hundreds of small factors pushing it one way or the other.