THIRD AMENDING LETTER TO CLINICAL RESEARCH AGREEMENT

Exhibit 10.55

March 6, 2006

Dr. Nicholas A Saccamano,

Senior Vice President,

Global Research Technology,

Pfizer Inc.,

50 Pequot Avenue,

New London, CT 06320

RE: Third Amending Letter of Agreement between Pfizer Inc. and Immunicon Corporation

Dear Dr Saccamano:

Immunicon Corporation (“Immunicon”) wishes to amend the agreement between Pfizer Inc. and its Affiliates (“Pfizer”) and Immunicon, executed on February 10, 2003 having a termination date of February 10, 2004, which was amended effective March 23, 2004 to terminate on February 10, 2005, and subsequently amended by a Second Amending Letter of Agreement dated December 12, 2004, to terminate the sooner of February 10, 2006 or the end of the clinical research study contemplated in the