who's responsibility is it?

I would guess it would depend on what you mean by adverse reaction and adverse reaction to what? A procedure? A med?

Do you mean, like, one of the listed reactions that can occure from a medication or do you mean adverce reacton such as, giving a med that the pt has a known allergy to?

It would depend if you knew and it was documented in the chart they had already had an adverse reaction to that drug or also to that class of drugs beforehand.

oh i mean adverse reaction to a med that was unknown

That is no one's fault/responsibility, if it is the first time and no one was aware of the problem. It is the responsibility of the first person who notes the problem to make everyone aware, and to do what needs to be done to correct the situation.