LIMITATIONS 45 



properties of the capsular polysaccharides. These sugars 

 are not toxic and do not seem to be responsible for the 

 accidents which accompany a pneumococcic infection. But 

 certain facts indicate that they can indirectly oppose the 

 normal mechanisms of defence against this disease. Indeed^ 

 by reason of the avidity with which they combine with the 

 antibodies, they tend to neutralize the substances resulting 

 from the processes of immunization, and thus prevent these 

 protecting agents from reaching the centres of infection. 

 What is more, the pneumococci enrobed in their shell of sugar 

 resist phagocytosis,^ whilst the naked microbes divested of 

 their capsules are energetically absorbed and destroyed by 

 the phagocytes. We have already seen that pneumococci 

 artificially deprived of their capsule lose their virulence and 

 do not multiply like the others. This is due in part to the 

 vigorous offensive of the phagocytes which devour their 

 enemies bereft of their protective armour. And now we 

 come to a most important point, the crowning touch to the 

 magnificent work which demanded such long years of effort 

 from the scientists of the Rockefeller Institute. 



The chemically purified capsular polysaccharides of the 

 pneumococci have lost the power to induce the formation of 

 antibodies in the animals to whom they are injected. In 

 other words, they cease to function as genuine antigenes whilst 

 conserving the faculty of combining with the specific anti- 

 bodies resulting from the injection of the integral microbe. 

 In this way, they are related to the important group of sub- 

 stances, immunologically active, which Landsteiner named 

 *haptenesV ^nd which behave in the same way. 



As these sugars, these carbohydrates, are antigenic when 

 they are accompanied by the microbic cell, Avery came to 



the conclusion that in this case they cannot exist as free 



^ Phagocytosis, which is the process by which certain wandering 

 cells of the organism surround, absorb, and sometimes digest any 

 invading element, was discovered by Metchnikoff. The 'phagocytes' 

 act as the poHcemen of the body. 



^ Haptenes are substances which react specifically, but fail to 

 induce immunity (i.e. to determine the appearance of antibodies) 

 when injected into an animal: they are not antigenic. 



