Disease Transmission and Mans Resistance 319 



The Dick Test 



This test was devised by George and Gladys Dick in 1924 as a 

 means of determining susceptibiHty of the individuals to scarlet 

 fever. The underlying principle is the same as that in the Schick 

 test, and mass surveys can be conducted during impending epi- 

 demics to determine susceptible and resistant individuals in a given 

 population. 



The Tuberculin Test 



In contrast to the Schick and the Dick tests the tuberculin reac- 

 tion is not an index of susceptibility. It is an allergic response to 

 specific proteins produced by Mycobacterium tuberculosis. Al- 

 lergy means altered reactivity, and the word is more or less 

 synonymous with hypersensitivity. Microbial antigens injected 

 into animals usually initiate the production of specific antibodies 

 which help to protect the recipient from the specific microbe or its 

 products. The introduction of some proteins, however, may cause 

 the animal to become sensitive (or hypersensitive) to that protein, 

 and upon subsequent injection of this antigenic material, an al- 

 lergic response may cause reactions severe enough to cause death 

 in some instances. When hypersensitivity is induced in animals 

 other than man, it is termed anaphylactic shock or anaphylaxis 

 —"against protection." 



Robert Koch prepared the first tuberculin in 1890, and this 

 crude material is called old tuberculin, in contrast to a modern 

 preparation known as purified protein derivative (PPD). By in- 

 jecting, or by rubbing into the skin, a minute amount of tuber- 

 culin, which is an extract produced from old cultures of Mycobac- 

 terium tuberculosis, a localized hypersensitivity manifests itself by 

 a reddening and swelling at the site of the injection, if the animal 

 is tuberculin positive. 



If a person has ever been infected with tuberculosis organisms, 

 he is allergic to the tuberculo-protein; even an old walled-off lesion 

 is capable of causing a positive skin reaction. Positive reactors, 

 particularly in the adolescent age group, should follow up thij 



