  Using the non-equilibrium Keldysh Green's function formalism, we investigate
the effect of defects on the electronic structure and transport properties of
two-dimensional topological insulators (TI). We demonstrate how the spatial
flow of charge changes between the topologically protected edge and bulk states
and show that elastically and inelastically scattering defects that preserve
the time reversal symmetry of the TI lead to qualitatively different effects on
the TI's local electronic structure and its transport properties. Moreover, we
show that the recently predicted ability to create highly spin-polarized
currents by breaking the time-reversal symmetry of the TI via magnetic defects
[Phys. Rev. B 93, 081401 (2016)] is robust against the inclusion of a Rashba
spin-orbit interaction and the effects of dephasing, and remains unaffected by
changes over a wide range of the TI's parameters. We discuss how the sign of
the induced spin currents changes under symmetry operations, such as reversal
of bias and gate voltages, or spatial reflections. Finally, we show that the
insight into the interplay between topology and symmetry of the magnetic
defects can be employed for the creation of novel quantum phenomena, such as
highly localized magnetic fields inside the TI.
