  We consider a loop mixing of two fermion fields of opposite parities whereas
the parity is conserved in a Lagrangian. Such kind of mixing is specific for
fermions and has no analogy in boson case. Possible applications of this effect
may be related with physics of baryon resonances. The obtained matrix
propagator defines a pair of unitary partial amplitudes which describe the
production of resonances of spin $J$ and different parity ${1/2}^{\pm}$ or
${3/2}^{\pm}$. The use of our amplitudes for joint description of $\pi N$
partial waves $P_{13}$ and $D_{13}$ shows that the discussed effect is clearly
seen in these partial waves as the specific form of interference between
resonance and background. Another interesting application of this effect may be
a pair of partial waves $S_{11}$ and $P_{11}$ where the picture is more
complicated due to presence of several resonance states.
