Rabbit alpha-fetoprotein: normal levels and breakage tolerance with haptenated homologous alpha-fetoprotein

Nanogram quantities of alpha-fetoprotein (AFP), a tumor associated fetal protein, were found in the serum of normal adult rabbits by radioimmunoassay. This AFP was isolated and shown to be immunologically identical to fetal AFP by immunodiffusion. Immunizations of rabbits with unmodified or desialylated AFP in complete Freund's adjuvant did not cause antibody formation, indicating the existance of tolerance against homologous AFP. The tolerance could be terminated by immunizing with hapten-coupled AFP, which resulted in production of rabbit antibodies reacting with unmodified rabbit AFP.