  Employing the closed-time path 2PI effective action (CTP 2PI EA) approach, we
study the response of an open interacting electronic system to time-dependent
external electromagnetic fields. We show that the 2PI EA provides a systematic
way of calculating the propagator and response functions of the system. Due to
the invariance of the 2PI EA under external gauge transformations, the response
functions calculated from it are such that the Ward-Takahashi hierarchy, that
ensures current conservation beyond the expectation value level, is satisfied.
These findings may be useful in the study of interacting electronic pumping
devices, and serve to clarify the connection between current conservation
(beyond the mean value level) and real-time nonlinear response theory.
