LIMITATIONS 43 



was a difficult one — was to isolate the substance in question 

 in a pure form, and to determine its chemical constitution 

 and its role in the immunological properties of the pneumo- 

 coccus as a whole. 



The capital discovery that these soluble substances, which 

 are responsible for type-specificity, belong to the family of 

 carbohydrates, that is to say sugars, is due to Dr. Michael 

 Heidelberger, a brilliant chemist and one of Avery's first 

 collaborators. No matter what type of pneumococcus they 

 are extracted from, they always possess in common the 

 chemical properties of the complex sugars, the polysac- 

 charides. But strangely enough, the polysaccharide derived 

 from each specific type of pneumococcus is chemically 

 different, and each one possesses particular properties which 

 distinguish it clearly from the others. What is more, 

 chemically purified solutions of these sugars manifest the 

 same specificity, from the immunological point of view, as 

 the microbes from which they are derived. To give an idea 

 of their astounding activity it suffices to mention the fact 

 that by means of the corresponding serum it is possible to 

 reveal their presence at a concentration of one five-millionth 

 or 0-0000002 grams in one cubic centimetre of solution.^ 



From the point of view of medicine, and especially of 

 serology and immunology, this discovery was of capital 

 importance for two reasons. First, because it had been 

 believed up till then that immunity reactions were solely due 

 to extremely complicated substances the chemistry of which 

 is very little known — the proteins. And second, because by 

 demonstrating that the capsular sugars were as chemically 

 distinct as they were serologically specific for each type of 

 pneumococcus, Avery and Heidelberger brought a striking 

 proof of the close relation existing between the chemical 

 constitution and the specificity of the microbes. 



^ Let us mention that Heidelberger and Avery accomplished the 

 same experiments, with equal success, on types A and C of Fried- 

 lander's bacillus. It is interesting to note that 75 litres of an auto- 

 lysed, eight-day-old culture medium are needed to yield about 

 I gram of polysaccharide. 



