FOKKER 100 ON APCH VECTORS HAS TCASII RA.

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WE WERE VECTORED FOR ILS RWY 27R AT PHL. ATC HAD BEEN ASSIGNING HDGS AND SPDS. WE WERE CLRED FOR THE ILS RWY 27R FROM 3000 FT. WE INTERCEPTED THE LOC AND SHORTLY THEREAFTER THE GS. APCHING THE SPEEZ LOM WERE HANDED OFF TO PHL TWR. AS WE TUNED THE TWR FREQ WE RECEIVED A TCASII TA. THIS WAS A VERY BUSY TIME AS WE WERE SPACED BEHIND ANOTHER ACFT ON THE APCH; CONFIGURING FOR LNDG; PERFORMING THE LNDG CHKLIST; AND COMPLYING WITH ATC SPD RESTRS. AS WE WERE DSNDING ON THE GS AND OVER SPEEZ; TCASII ANNOUNCED THE RA 'REDUCE DSCNT.' BY NOW WE HAD VISUALLY ACQUIRED BOTH INTRUDER ACFT DISPLAYED BY TCASII. IN RESPONSE TO THE RA THE AUTOPLT WAS DISCONNECTED AND WE DEVIATED SLIGHTLY ABOVE THE GS. TCASII SOON ANNOUNCED 'CLR OF CONFLICT' AND WE EASILY RETURNED TO THE GS AND LANDED WITHOUT FURTHER INCIDENT. BECAUSE OF OUR WORKLOAD AND THE MINOR NATURE OF THE DEV THE RA WAS NOT RPTED TO ATC. I DID RPT THIS INCIDENT TO MY DISPATCHER AND THE COMPANY AND WAS TOLD THAT THEY WOULD ADVISE ATC OF THE EVENT. HAD IT NOT BEEN FOR TCASII THIS ENCOUNTER WOULD HAVE BEEN VERY CLOSE. ATC DID NOT ADVISE US OF THIS TFC. IF THERE IS A VFR CORRIDOR THROUGH THE CLASS B AIRSPACE AT THIS LOCATION; I WOULD SUGGEST MOVING IT BEFORE SOMEONE GETS HURT. AT THE VERY LEAST ALL TFC SHOULD RECEIVE AN ADVISORY FROM ATC.