Desense is typically caused by an off-channel signal that is sufficently powerful that it causes the receiver to cut-back the gain (Automatic Gain Control, AGC) and thus you can't 'hear' the on-channel signal.
At each subsequent RF or Intermediate Frequency (IF) stage, the bandwidth typically becomes narrower and narrower. The AGC loop must control the RF and IF gain closer to the front end (and thus the AGC circuit is exposed to a wider bandwidths than the entire receiver end-to-end). Thus, an off-channel signal can affect the AGC, but you don't hear the off-channel signal because it isn't within the bandwidth of the whole chain. But the off-channel signal does cause the receiver to go deaf due to the action of the AGC.
Another possible mechanism is saturation, but the AGC mechanism is more likely since it can occur at more reasonable amplitudes. Except with digital radios (those with DSP in the signal path) - they need lots of bits (bit depth) to accomodate weak signals accomanied by off-channel stronger signals - otherwise 'digital saturation'. 16 bits is usually inadequate for such application.
