872 IMMUNOLOGY 



infections. This work has yielded many facts of great interest, but ac- 

 tual practical applications have been limited. 



ARTIFICIAL IMMUNIZATION 



The earlier literature on this subject has been critically reviewed by 

 W. H. Taliaferro (1929), to which publication the reader is referred 

 for details. Only a few of the more successful examples are cited. 



The greatest success with artificial immunization has been attained 

 in Babesia infections of cattle and consists of inducing in young healthy 

 animals a low-grade or latent infection which is frequently controlled 

 with drugs. During this latent infection, the animal possesses a solid 

 immunity to superinfection, similar to the condition in malaria. Like 

 malaria, however, the host's defenses may weaken and permit severe 

 and even fatal relapses. 



Mention has been made of the fact that one attack of oriental sore 

 in man generally confers a lasting immunity. As the natural sores oc- 

 cur on the face or other exposed portions of the body and leave dis- 

 figuring scars, it has been the practice in many endemic centers for 

 centuries to inoculate children on unexposed portions of the body. In 

 a sense this is the crudest type of immunization, in that the highly viru- 

 lent virus is employed to induce the ordinary disease. The use of at- 

 tenuated organisms has not met with particular success. 



Several investigators have been able to immunize laboratory animals 

 with dead trypanosome vaccines. So far, however, such vaccines have 

 not been extensively applied in a practical way and the outlook is 

 not favorable. Among other difficulties, the attainment of an ade- 

 quately polyvalent vaccine can hardly be hoped for, owing to the exist- 

 ence of so many immunologically different strains of trypanosomes. 



IMMUNOLOGICAL REACTIONS USED IN DIAGNOSIS 



Considerably more success has followed the practical application of 

 immunological reactions in diagnosis than in immunization, but even 

 here the success has been limited. This is due in part to the technical 

 difficulty of perfecting the tests, especially when only weak reactions 

 ensue, and in part to the fact that they have to be as satisfactory as or 

 better than the demonstration of the parasites, which has been rendered 

 remarkably delicate in certain blood infections, notably malaria and 



