The adaptive immune system is highly capable and uses a range of techniques to identify and eliminate molecules it has no prior experience with.

In one example, the mirror molecules would likely cause some sort of localized inflammation or other lesion, and then the immune system would be attracted to the area, generating trillions of combinations of randomized protein sequences. If within that trillion there is just 1 antibody that can bind to the mirror life, the immune system will duplicate that molecule many times, and they will bind to the mirror molecules, recruit other proteins that will isolate and degrade them, and then remove them from the body.

Due to the complexity of protein structures, antibodies can bind to mirror molecules.

Every time I show a computer scientist this system: https://en.wikipedia.org/wiki/V(D)J_recombination they remark, "Wait, nature uses self-modifying code? It generates trillions of random combinations, shuffles them around, finding the best ones, and then amplifies them? You know that sort of sounds like this CS technique called genetic algorithms" at which point you just stare at them until the exclamation point above their head goes "BINGO"