  We derive the expression for the nonperturbative coupling of a hadron to two
external currents from its underlying structure in QCD. Microscopically, the
action of each current is resolved to a coupling with dressed quarks. The
Lorentz structure of the currents is arbitrary and thereby allows to describe
the hadron's interaction with photons as well as mesons in the same framework.
We analyze the ingredients of the resulting four-point functions and explore
their potential to describe a variety of processes relevant in experiments.
Possible applications include Compton scattering, the study of two-photon
effects in hadron form factors, pion photo- and electroproduction on a nucleon,
nucleon-pion or pion-pion scattering.
