  Taking into account that neutrinos are massive particles and that they are
produced mainly as states of negative helicity, we show that the neutral and
charged current interactions change these neutrinos into transversally
polarized states. This implies a considerable reduction of the neutrino flux
when propagating through ordinary matter (electrons, protons and neutrons). The
same happens when neutrinos propagate through the sea of relic neutrinos if
these neutrinos are degenerate fermions. However, in this case the change of
helicity depends on the value of the neutrino--anti-neutrino asymmetry.
