  Starting from relativistic quantum field theories, describing interacting
nucleons and pions coupled to the dynamical electromagnetic field, the pion
degrees of freedom are eliminated by means of functional integration. Apart
from taking into account some operators perturbatively in $e$, e.g. the vacuum
polarization, this procedure is exact, giving effective theories for nucleons
and photons. The subsequent nonrelativistic reduction yields the corresponding
nonrelativistic quantum field theory. The latter is unique, irrespective of the
precize form of the original nucleon-pion interaction. Nucleonic potentials and
electromagnetic interactions are mutually consistent. Local gauge invariance is
satisfied at any stage of the formal developments.
