Immunicon ( IMMC) has extended a research and development agreement with Pfizer ( PFE) under which the companies are developing new reagents designed to detect antigens on circulating tumor cells. The collaboration began in February 2003. A new Phase I trial has been added to the agreement and will begin enrolling participants. An antigen is any substance that causes the body's immune system to go into action. Antigens can come from the environment, for instance chemicals, viruses or pollen, or can be formed within the body, in the case of some bacterial toxins or tissue cells. Immunicon, based in Huntingdon Valley, Pa., is developing human diagnostic and life-science research products, with an initial focus on cancer management. Shares of Immunicon were gaining 28 cents, or 7.8%, to $3.89 Tuesday morning.