It is comparatively easy to immunize laboratory animals against a virulent typhoid bacillus by injecting them intraperitoneally or subcutaneously with gradually increasing doses of young, living cultures. These animals soon become resistant toward many times the minimum fatal dose of the organism, and their sera acquire high agglutinative and bacteriolytic properties. Small doses of serum of the immunized animals protect other animals against several times the minimum fatal dose of typhoid cultures, even if the serum is injected several hours after the inoculation.
Although the laboratory experiments are very encouraging, it has been found that the clinical results from these antityphoid sera are practically nil. Stokes and Fulton, for instance, immunized a hog and got very good results in laboratory animals with this serum. Half a cubic centimeter protected a guinea-pig against seven times the minimum lethal dose of a virulent culture. They treated five typhoid fever patients with this