Opposite parity fermion mixing and baryons $1/2^{\pm}$

We develop a variant of $K$-matrix, which includes the effect of opposite parity fermions (OPF) mixing, and apply it for description of $\pi N$ partial waves $S_{11}$ and $P_{11}$. OPF-mixing leads to appearance of negative energy poles in $K$-matrix and restoration of MacDowell symmetry, relating two partial waves. Joint analysis of PWA results for $S_{11}$ and $P_{11}$ confirms significance of this effect.