  We calculate the interaction kernel K for two-dimensional diffusive
electrons. The screening of the Coulomb interaction together with the Fermi
statistics induces a spin selection rule for electron-electron scattering so
that in leading order in the inverse conductance only pairs of electrons with
antiparallel spins do scatter. At low temperature, this results in a larger
coherence length for fully polarized electrons and thus in a positive in-plane
magnetoresistance. An applied in-plane magnetic field also induces a
nonmonotonous behavior of K at finite temperature. Alternatively, the vanishing
of the scattering in the triplet channel strongly reduces ferromagnetism deep
in the metallic regime. These effects weaken as the density of charge carriers
is reduced.
