What is the subject of the following sentence?

Food allergies are adverse reactions to an otherwise harmless food or food component that involves an abnormal response of the body's immune system to specific protein(s) in foods.

This is a beginning sentence of a paper on food allergies. I was wondering what the subject of the verb involves and have asked opinions from three English language experts who also have scientific knowledge, but their opinions vary. Their answers were as follows:

Person 1: The subject is food allergies and the verb should be involve.

Person 2: The subject is abnormal response and the verb should be involve.

Person 3: The subject is food or food component, so this sentence is correct. He also pointed out that the paper is written in British English (I showed him the following sentences and he got some clues from there) so here involves means evokes or causes.

Which of these opinions is the correct one?

I think your Person 2 is supposed to say that the subject of the verb "involve" is "adverse reactions." I agree with the example that tchrist says, although the "Person 2" example wouldn't make sense. I think you should edit your question title to ask for the subject of the verb, because the subject of the sentence is obviously "food allergies."

And scientifically, that would make the most sense to me. It's not usually the food that's the cause of the problem, but the body's improper immune response to it. The body is normally able to discriminate between food (which should have no immune response) and bacteria, fungi, and other bad things (which should be attacked by your mucosal immune system). But you were asking about grammar, not biology...

I put this in a comment but also want to add it here. The next sentence from the original FAO report says: "True food allergies may involve several types of immunological responses." I think that makes it unlikely that the scientists intended to use the definition in the third example.

I vote for number two. I think the sentence is fundamentally trying to say:

Allergies are reactions that involves a response.

Which if my reading is correct, means that involves needs to be involve.

The question asks for the subject of the sentence

Food allergies are adverse reactions to an otherwise harmless food or food component that involves an abnormal response of the body's immune system to specific protein(s) in foods

The subject of this sentence is Food allergies, which is a plural noun phrase, and agrees with the verb are, which is also plural.

However, the rest of the question appears to refer to the relative clause

that involves an abnormal response of the body's immune system to specific protein(s) in foods

which follows and modifies the noun component. This is not the main clause, and is not a sentence. However, it is a clause and it has a subject.

The subject is that; as a relative pronoun, it can be singular or plural, depending on its antecedent. In this case, the antecedent of that is component, which is singular, and agrees with the singular verb involves.

You have to distinguish clauses from one another before you can talk about sentences usefully.