22 ACTIVE IMMUNIZATION. 



2. COCCIDIOSIS gives changes in the liver due to the settling of the coccidi ova forms. 

 The parasites are easily recognized, with the microscope, as present in the pus. 

 Following labor, guinea-pigs are very susceptible to sepsis. 

 IN RATS, trypanosomiasis is of frequent existence, but is not pathogenic. 



The antigens are injected either subcutaneously , intraperito- 

 The Tech- neaMy or intravenously. Only on exceptional occasions is 

 niqueof Active another entrance path chosen. 



Immuniza- As regards the amount to be injected, one cannot, very well, 

 give general rules. It is important to prevent severe re- 

 actions, although the question is still a disputed one, whether 

 marked reactions tend to produce a better immunity. It is certain, how- 

 ever, that inoculations of antigens in such minute doses as to apparently 

 give no reaction, can still lead to immunity and the production of antibodies. 

 Occasionally a single injection suffices for immunization. Repeated in- 

 oculations are usually necessary, especially so when a "highly valent" serum 

 is desired, i.e., one containing a great number of antibodies or having high 

 protective properties. 



When repeated inoculations are undertaken, there are various methods of 

 procedure. 



i a. A small dose of antigen is injected. If a reaction sets in, one waits 

 until this reaction has entirely subsided, then (not before the fifth day) the 

 second injection a somewhat larger dose is given. After an interval of 

 ' 5 to 8 days, a third injection of a still higher dosage is administered, and 

 so on, again. 



i b. The intervals are the same, but the amounts of antigen remain the 

 same at each injection. 



Both of these methods give excellent results and therefore are most 

 frequently used. 



2. For several successive days, a small or medium dose of antigen is 

 injected. Each injection produces only a slight reaction. 



This last scheme according to Fornet is especially suitable for the gain- 

 ing of precipitation sera. As is evident, it has the advantage of gaining 

 the immunity rapidly. 



3. Inoculations are given at very long intervals (intermissions of four 

 weeks or more). This method produces good sera, but has the disadvan- 

 tage of requiring too long a time. 



The methods of active immunization can also be divided according to 

 the nature of the antigen. 



1. Immunization with a living virus, 



2. Immunization with a dead virus, 



3. Immunization with bacterial extracts, 



4. Immunization with bacterial toxins. 



