50 ohm antenna matching

Hi guys... wanna know does a receiver front end requires additional 50Ω matching network, if the antenna feeding it is 50Ω terminated? if yes than doesn't it effect the selectivity...?

To be able to measure various impedances, there is a general concensus to use 50 Ohms for signal sources, transmission lines and loads.
In your case the receiver input may be matched (i.e. having the impedance of ) 50 Ohms as well as the antenna. Antenna impedance often differs (due to conductive objects in its vicinity like ground), so a matching device may improve the impedance matching.
Sensitivity is defined only for the receiver. The complete system, antenna plus the receiver, can achieve different kind of sensitivity. Please refer to low-noise microwave systems where more details are studied.