  The leading order correction to the metric of a Schwarzschild black hole, due
to the backreaction of infalling fermionic matter fields, is shown to produce a
shift of the event horizon such that particles that would constitute Hawking
radiation at late retarded times are now trapped. Fermionic field operators
associated with infalling and outgoing modes at the horizon behave canonically
in the semiclassical approximation. They are, however, shown to satisfy a
nontrivial exchange algebra given in terms of the backreaction, when the shift
is `quantized' by means of correspondence. The consequent exchange algebra for
bilinear fermionic densities is also obtained.
