  The cross section for polarized and unpolarized electron-proton scattering is
calculated taking into account radiative corrections in leading and next-to
leading logarithmic approximation. The expression of the cross section is
formally similar to the cross section of the Drell-Yan process, where the
structure functions of the electron play the role of Drell-Yan probability
distributions. The interference of the Born amplitude with the two photon
exchange amplitude (box-type diagrams) is expressed as a contribution to
K-factor and it is larger when the momentum is equally shared between the two
photons, assuming that proton form factors decrease rapidly with the momentum
transfer squared. The calculation of the box amplitude is done when the
intermediate state is a proton or the Delta-resonance. The results of numerical
estimations show that the present calculation of radiative corrections can
bring into agreement the conflicting experimental results on proton
electromagnetic form factors an that the two photon contribution is very small.
