Georg von der Gabelentz, Henry Sweet, and Philipp Wegener have all written on the subject of sentence words. Both Sweet and Wegener include yes and no in this category, with Sweet treating them separately from both imperatives and interjections, although Gabelentz does not.
Watts classifies yes and no as grammatical particles, in particular response particles.
He also notes their relationship to the interjections oh and ah, which is that the interjections can precede yes and no but not follow them. Oh as an interjection expresses surprise, but in the combined forms oh yes and oh no merely acts as an intensifier; but ah in the combined forms ah yes and ah no retains its standalone meaning, of focusing upon the previous speaker's or writer's last statement. The forms *yes oh, *yes ah, *no oh, and *no ah are grammatically ill-formed. Aijmer similarly categorizes the yes and no as response signals or reaction signals.