Abstract : The Maxwell-Bloch system describes a quantum two-level medium interactingwith a classical electromagnetic field by mediation of the the populationdensity. This population density variation is a purely quantum effect which isactually at the very origin of nonlinearity. The resulting nonlinear couplingpossesses particularly interesting consequences at the resonance (when thefrequency of the excitation is close to the transition frequency of thetwo-level medium) as e.g. slow-light gap solitons that result from thenonlinear instability of the evanescent wave at the boundary. As nonlinearitycouples the different polarizations of the electromagnetic field, theslow-light gap soliton is shown to experience effective scattering whithcharges in the medium, allowing it for instance to be trapped or reflected.This scattering process is understood qualitatively as being governed by anonlinear Schroedinger model in an external potential related to the charges(the electrostatic permanent background component of the field).