Immunoconjugates for impeding weight gain and treating obesity in a subject are disclosed. The immunoconjugates comprise a ghrelin mimetic polypeptide hapten, a spacer moiety comprising one of more polyethylene glycol (PEG) units, and a protein carrier moiety. Immunoconjugates optionally include a conjugation moiety for conjugating the polypeptide hapten with a linker moiety or the protein carrier moiety and a linker moiety for conjugating the conjugation moiety with the protein carrier moiety.