  General properties of the differential cross section and of the polarization
observables are derived for the $e^++e^-\to N+\bar N$ reaction, in presence of
two--photon exchange. Polarization effects are investigated for longitudinally
polarized electron and polarized antinucleon and/or polarized nucleon in the
final state. The single--spin asymmetry induced by the transverse polarization
of the electron beam is also discussed.
