  Helical edge modes of 2D topological insulators are supposed to be protected
from time-reversal invariant elastic backscattering. Yet substantial deviations
from the perfect conductance are typically observed experimentally down to very
low temperatures. To resolve this conundrum we consider the effect of a single
magnetic impurity with arbitrary spin on the helical edge transport. We
consider the most general structure of the exchange interaction between the
impurity and the edge electrons. Moreover, for the first time, we take into the
account the local anisotropy for the impurity and show that it strongly affects
the backscattering current in a wide range of voltages and temperatures. We
show that the sensitivity of the backscattering current to the presence of the
local anisotropy is different for half-integer and integer values of the
impurity spin. In the latter case the anisotropy can significantly increase the
backscattering correction to the current.
