A lighting unit including devices for emitting a luminous flux in the direction of a reflector featuring devices for optical deflection of the flux toward the outlet of the unit, the reflector including a support wall that is fastened to the optical deflection devices and is disposed across the optical deflection devices so as to be disposed between the optical deflection devices and the emission devices.The support wall includes a masking portion that is disposed facing the emission devices and a so-called cut-off edge of which is cut away so as to form a mask integral with the support wall adapted to allow to pass only a portion of the rays emitted by the emission devices. The invention also concerns a motor vehicle headlamp, notably in that it includes two lighting units disposed transversely side by side.