  We present, for the first time, an \textit{ab initio} calculation of the
individual up, down and strange quark helicity parton distribution functions
for the proton. The calculation is performed within the twisted mass
clover-improved fermion formulation of lattice QCD using one ensemble of
dynamical up, down, strange and charm quarks with a pion mass of 260 MeV. The
lattice matrix elements are non-perturbatively renormalized and the final
results are presented in the $\overline{ \rm MS}$ scheme at a scale of 2 GeV.
We give results on the $\Delta u^+(x)$ and $\Delta d^+(x)$, including
disconnected quark loop contributions, as well as on the $\Delta s^+(x)$. For
the latter we achieve unprecedented precision compared to the phenomenological
estimates.
