172 VETERINARY BACTERIOLOGY 



are made as in the preceding. It is possible there may sometimes 

 be intrinsic differences in these leukocytes, which render direct 

 comparison between their activity somewhat misleading. These 

 difficulties are more than outweighed by the increased ease of 

 manipulation, and good results are commonly secured. 



The variation in the opsonic content of a patient's blood gives 

 valuable information relative to the development of resistance. 

 As stated above, in many chronic infections the opsonic index 

 is below unity. In the treatment of tuberculosis in the human, 

 by means of minute injections of tuberculin, the determination 

 of the opsonic index has given much information of practical 

 nature. The injection of bacterial vaccines or bacterins which 

 have been carefully studied is followed at first by an initial lower- 

 ing of the index. This is called the negative phase. Later, the 

 index should rise above unity, when the positive phase is said to be 

 established. If the index can be kept above unity, the prognosis 

 is generally regarded as favorable. 



Methods of Opsonic Immunization. Immunization due to 

 increased opsonin is generally brought about by the introduction of 

 bacteria or their products into the body. Such an immunity is, 

 therefore, active. There is good reason to believe that injection 

 of an opsonic serum in some cases confers some passive immunity. 

 The most commonly practised method of increasing the opsonin 

 content of the blood is by vaccination. This may be defined as 

 the injection or inoculation with organisms or their products in 

 such a way that the disease produced will run a benign course. 

 Such a procedure in many cases induces the production of bac- 

 teriolysins, in others opsonins, and in still others both. Vaccines 

 may consist of either living or dead organisms. If the former, 

 they may be either virulent or attenuated. Vaccination with viru- 

 lent organisms is not frequently practised. Some organisms do 

 not produce typical and serious diseases unless they enter the 

 body by a certain channel. The injection of the Asiatic cholera 

 organism subcutaneously is not followed by any serious results, 

 but, when the alimentary tract is the infection atrium, the char- 

 acteristic symptoms of the disease are produced. This fact has 

 been used as the basis for practical vaccination against this dis- 

 ease. Usually vaccines consist of attenuated organisms. Atten- 



