Semantic features
The characteristic features which distinguish interjections from practically all other words lie in a different sphere. The interjections, as distinct from nouns, verbs, prepositions, etc., are not names of anything, but expressions of emotions.
Syntactic features
The usual interpretation is that the interjection stands outside the structure of the sentence. Another view is that it is syntactically a kind of parenthesis at least in some cases. The controversy cannot be decided by objective investigation and the answer only depends on what we mean by sentence structure on the one hand, and by some element or other being outside the sentence structure, on the other.