https://www.creative-biolabs.com/native-antibody/immunogen-preparation.htm

An immunogen is an antigen or any substance that can specifically bind to components of the immune system (antibodies, lymphocytes). The term antigen is derived from its ability to induce antibody production. Although all antigens are recognized by specific lymphocytes or antibodies, not every antigen can evoke an immune response. Those antigens that can induce an immune response are called immunogens. Therefore, the immunogen can be defined as a complete antigen composed of macromolecular carriers and epitopes (determinants) that can induce an immune response.