TOXINS AND ANTITOXINS 255 



Schick Test for Susceptibility. — In 1913 Schick announced that 

 b}^ means of the Romer technique he could determine susceptibility 

 or immunity to diphtheria. He found that susceptible individuals 

 do not possess sufficient circulating antitoxin to neutralize 1/40 

 or 1/50 of a guinea pig M.L.D. of toxin when it is injected into 

 the outer layers of the skin (intradermally) of the forearm. A 

 positive reaction indicating susceptibility develops witliin IS to 

 26 hours at the site of inoculation and persists for 7 to 15 days. 

 Occasionally delayed reactions are encountered. The positive 

 reaction manifests itself as an area of hyperemia (redness) one 

 centimeter or more in diameter. Also there may be some swelling 

 and induration. As the reaction subsides scaling manifests itself 

 and there remains a brownish pigmentation. A negative reaction 

 indicates that the individual's blood contains at least 1/40 of a 

 unit of antitoxin per cubic centimeter. The test is performed by 

 injecting into the outer layers of the skin a toxin solution of such 

 strength that 0.1 c.c. or 0.2 c.c. contains 1/40 or 1/50 of a guinea 

 pig M.L.D. Since some individuals are hypersensitive to toxin 

 (see NeiU, Fleming, Sugg and Gaspari, 1930), it is quite desirable 

 to run controls with toxin heated to 60°-75° C. for thirty minutes. 

 Pseudoreactions commonly disappear within 2 or 3 days (Banzhaf, 

 1928) and occasionally they are confusing. The time of disap- 

 pearance and lack of pigmentation are frequently used as criteria 

 in interpreting a reaction as negative rather than the use of a 

 control. 



Recently Taylor and Moloney (1939) recommended that the 

 Schick toxin be prepared from "fresh" toxin less than one year 

 old and free from preservatives. They also suggest that the new 

 Schick toxin have twice the toxicity and lower combining power 

 than the standard Canadian dose. They insist that this will in- 

 crease the percentage of interpretable reactions. Their claims 

 seem to be substantiated by Cameron and Gibbard (1941). 



Variation in Susceptibles in Urban and Rural Populations. 

 — According to data collected by Park (1919, 1933) and others, the 

 number of susceptible individuals in a population varies in dif- 

 ferent age groups and also with the opportunity afforded by the 

 environment for acquiring immunity from mild contact infec- 

 tion. He found that at birth 15 per cent of the infants born in 



