256 IMMUNOLOGY 



the cities are positive as contrasted with 40 per cent of those born 

 in rural communities. Susceptibility in the other aoe oroups he 

 tabulates in Table VIII. 



Table VIII* 



♦Parks and Williams : Pathogenic Microorganisms, Lea & Febiger, publishers. 



Susceptibility in Young Adults. — In a series composed of 

 medical students at the University of Kansas and falling within 

 the age groups of nineteen to twenty-three years, there were 28 per 

 cent Schick positive individuals. This represents a mixed group 

 made up of students coming from cities and rural communities. 

 It will be noted from the data in the above table that ages of 

 greatest susceptibility are the second, third and fourth years of 

 childhood. 



Susceptibility Determined by First Dose of T.A.T. — In 1923, 

 Park noted that susceptible individuals react to a subcutaneous 

 injection of 1 c.c. of a toxin-antitoxin mixture, having a standard 

 toxicity, with a reaction similar to that obtained by the Schick 

 test. Thus a susceptibility test and the first of three immunizing 

 injections are combined. It is obvious that this has distinct ad- 

 vantages. Since the reaction depends upon the presence of diph- 

 theria toxin, one can readily appreciate why toxoid, which is 

 nontoxic, cannot be used in any susceptibility test, although it is a 

 good immunizing agent. The test suggested by Park is read on 

 the fifth or six day just as is done with a Schick test. The pseudo- 

 reactions are usually gone by the third day. Park says that the 

 highest percentage of pseudoreactions develop in older children 

 and adults. Controls are usually not run with this test. 



Active and Passive Immunity. — As mentioned earlier in this 

 chapter, interest in active and passive immunity to diphtheria 

 began with Behring's and Kitasato's discovery that antitoxins can 



