An immunogen is the substance used to elicit an immune response and raise an antibody. It is not the same as an antigen: every immunogen is an antigen, but many antigens — short peptides among them — are too small to provoke a response on their own. Such molecules are haptens, and they need help.
Carriers and why the immunogen sequence is the key datasheet field
Peptides below roughly 5 kDa are conjugated to a carrier protein: KLH (keyhole limpet haemocyanin) is preferred for immunisation because it is large, highly immunogenic and phylogenetically distant from mammals, while BSA or ovalbumin is used for the screening plate so that anti-carrier antibodies are not mistaken for anti-peptide ones. Conjugation chemistry matters too — glutaraldehyde, EDC or maleimide via an added terminal cysteine — because the coupled terminus is largely hidden from the immune system and antibodies will be biased toward the free end.
Practically, the immunogen sequence is the single most predictive field on an antibody datasheet. It tells you which epitope is likely recognised, which analogues will cross-react, and whether the same sequence is conserved in your species of interest. The matching free peptide is sold as a blocking peptide.
Related terms and material
antiserum · ELISA. Reference material: antibodies, peptide libraries.