ACTIVE IMMUNITY 455 



(i) It may kill the patient, or if the patient recovers (2) the disease may 

 not only confer immunity but may actually create a predisposition for 

 recurrence as in erysipelas and influenza; (3) where one attack confers 

 no evident protection against a second, as gonorrhea; (4) the immunity 

 conferred is of short duration, as cholera; (5) a lasting protection against 

 subsequent attacks is possessed by those recovered from plague and 

 typhoid fever. 



Those serotherapeutic products classified under active immunity 

 are called "antigens." The word "antigen" means "former of antibodies" 

 and when an antigen is administered parenterally (that is, by the subcu- 

 taneous, intramuscular, intraneural, intraspinal, or intravenous route, in 

 order to reach tissues not directly accessible) its presence in the body will 

 stimulate the production by the tissues of true antibodies. It usually 

 requires several days to build up an immunity, but when obtained is 

 quite permanent and may even last for years. To the class of antigens 

 belong the vaccines, viruses, tuberculins, toxins and bacterins. 



Artificial active immunity is brought about in four different ways 

 as follows: 



1. BY INOCULATION OF VIRULENT ORGANISMS OR TOXINS. This 

 method is principally used in experimental work. Formerly vaccination 

 against small-pox was made with virus obtained directly from the diseased, 

 but this practice has been long discontinued. Most of the serums used 

 in passive immunity are obtained by this method and will -be fully dis- 

 cussed under "Prophylactic Passive Immunity." Kitt's method of 

 vaccination against symptomatic anthrax and in immunization of rattle- 

 snake venom was accomplished by inoculation of the bacteria or toxin 

 directly into the blood stream. 



2. BY INJECTION or ATTENUATED ORGANISMS, VIRUS OR TOXIN. 

 There are several methods of attenuation used, and no single method is 

 suitable for all organisms. Animal passage is an important one and is 

 surprisingly variable in its results. For instance, passing the bacillus 

 of swine erysipelas through the rabbit several times increases its virulence 

 for the rabbit but decreases it for the swine, while passing the organism 

 through the dove increases its virulence for swine (Pasteur). Attenua- 

 tion may also be accomplished by exposure to air and light, as chicken 

 cholera (Pasteur); by cultivation of the organism at high temperatures, 

 as anthrax (Pasteur); by chemical agents, as diphtheria toxin (Behring 

 and Roux); by desiccation, as rabies (Pasteur). 



Vaccine virus and antirabic virus are the two most notably successful 

 examples of this class. 



VIRUS VACCINICUM, U.S.P. The pustules of vaccina or cowpox 

 removed, under aseptic conditions, from vaccinated animals of the 

 bovine species. The vaccine pulp, after being thoroughly rubbed up in a 

 mortar or passed through a grinder, then strained to remove coarse 



