The development of immunity against pathogenic bacteria, viruses and spontaneously occurring tumors requires the concerted activation of a number of humoral and cellular players. The interest of our laboratory is focused on the characterization of the mechanisms that mediate activation, proliferation and differentiation of CD4+ T lymphocytes, because of the central role that these cells play during the development of a protective immune response. In particular we have established an animal model that allows the investigation of antigen-and tumor-specific CD4+ T lymphocytes both in vivo and in vitro.