20 ACQUIRED IMMUNITY 



to smallpox of a perfectly healthy person is probably not great, 

 whereas that produced by an attack of the disease or by vacci- 

 nation is for a time almost absolute. Yet all degrees of acquired 

 immunity exist, from the very slight amount which is developed 

 during an attack of pneumonia, and which is probably only just 

 sufficient to cut short the disease, to the enormous degree that 

 can be obtained in animals hyperimmunized to diphtheria or 

 tetanus toxin or hypervaccinated to B. typhosus. Perhaps our 

 conception of immunity in the past has been influenced too 

 strongly by a study of these latter conditions, which are readily 

 induced in the laboratory, but rarely if ever seen in the actual 

 practice of medicine. They represent in an extreme form the 

 changes which follow disease of natural origin, and possess the 

 theoretical interest which attaches to all extreme cases. 



ACQUIRED IMMUNITY occurs in two distinct forms active and 

 passive. A third form exists, which we may call mixed, since it is 

 brought about by a combination of the procedures necessary for 

 the development of the other two. 



Active immunity may be defined as acquired immunity, due to 

 an attack of the disease in question in its normal form, or in a 

 modified and less severe form of artificial production. The 

 essential feature is that the cells and tissues of the person or 

 animal should be subjected to the action of the bacterium (or its 

 toxin), and by its own efforts, and as a result of an active struggle 

 with it, should become less susceptible to its toxin than before. 

 Active immunity is developed only as a result of an illness of the 

 host, due to the action of the microbe on its cells ; and this illness 

 may be of any degree of severity, ranging from an unmodified 

 attack of the disease which may threaten life down to the most 

 transitory and unimportant reaction due to an injection of a 

 minute dose of a mild vaccine. And one of the great aims of 

 modern preventive medicine is to reduce the severity of the disease 

 necessary to produce acquired immunity to a minimum. The 

 greatest step ever made in this direction was Jenner's substitution 

 of vaccination for inoculation. In each case the effect is the same 

 as regards the resulting immunity (though in different degree), 

 but the disease in the former case is mild and devoid of danger, 

 in the latter severe and dangerous. As a general rule, it may be 

 taken that the severer the disease the stronger and more lasting 

 the acquired immunity. A good example will be quoted when 

 dealing with mixed immunity. This is not necessarily the case, 



