Circulating antitoxin in rabbits after ingestion of diphtheria toxoid.

Immune responses following antigen ingestion vary from stimulation to suppression depending on animal species, antigen, and experimental protocol. Young adult rabbits were given either 0.02% diphtheria toxoid or 0.1% bovine serum albumin in drinking water for 10-day periods, a protocol previously found to immunize human infants fed bovine serum albumin. Specific serum antibody was detected by radioimmunoassay in 10 of 13 rabbits fed diphtheria toxoid for 10 days and 11 of 13 rabbits fed bovine serum albumin. Response to oral diphtheria toxoid in some animals was equal to that after an injection of alum-precipitated diphtheria and tetanus toxoids. All rabbits fed a second course of either antigen showed an enhanced, rapid rise in antibody concentration. Avidity of the antibody also increased markedly, suggesting that priming and memory had been established during the first feeding. Similar anamnestic responses occurred in animals given a parenteral challenge after ingesting antigen. One month after administration of diphtheria toxoid, no significant difference in serum antibody levels was present between unfed animals challenged with diphtheria-tetanus toxoids and although avidity of the antitoxin was higher in fed animals. Antibody activity could be detected only in the immunoglobulin G serum fractions with either antigen. These responses suggest that oral immunization protocols using protein antigens can be adapted for use against toxins causing disease.