  We theoretically describe the spin excitation spectrum of a two dimensional
electron gas embedded in a quantum well with localized magnetic impurities.
Compared to the previous work, we introduce equations that allow to consider
the interplay between the Coulomb interaction of delocalized electrons and the
$sd$ exchange coupling between electrons and magnetic impurities. Strong
qualitative changes are found : mixed waves propagate below the single particle
continuum, an anticrossing gap is open at a specific wavevector and the kinetic
damping due to the electron motion strongly influences the coupling strength
between electrons and impurities spins.
