  Towards understanding the multi-orbital quantum antiferromagnetism in iron
pnictides, effective spin couplings and spin fluctuation induced quantum
corrections to sublattice magnetization are obtained in the $(\pi,0)$ AF state
of a realistic three band interacting electron model involving $xz$, $yz$, and
$xy$ Fe 3d orbitals. The $xy$ orbital is found to be mainly responsible for the
generation of strong ferromagnetic spin coupling in the $b$ direction, which is
critically important to fully account for the spin wave dispersion as measured
in inelastic neutron scattering experiments. The ferromagnetic spin coupling is
strongly suppressed as the $xy$ band approaches half filling, and is ascribed
to particle-hole exchange in the partially filled $xy$ band. The strongest AF
spin coupling in the $a$ direction is found to be in the orbital off diagonal
sector involving the $xz$ and $xy$ orbitals. First order quantum corrections to
sublattice magnetization are evaluated for the three orbitals, and yield a
significant $37\%$ average reduction from the Hartree-Fock value.
