Why Desensitization happens when a strong nearby signal, usually off frequency, drives the receiver's front end toward compression so the gain available for the wanted weak signal drops. Inserting attenuation ahead of the first RF amplifier lowers the interfering signal back into the linear region of the front end, and the desired signal drops by the same amount, so the signal-to-noise ratio is usually preserved if the receiver still has enough sensitivity. A preselector or bandpass filter does the same job selectively by rejecting the offending out-of-band signal.
Watch out Increasing front-end gain is exactly backwards: more gain drives the mixer and RF stage into compression sooner and makes desensitization worse. AGC time constants and IF frequency affect audio recovery and image rejection, not front-end overload.
Desense is overload: turn the front end down, not up. Attenuator or preselector first, never more gain.