IMMUNITY. 55 



Phagocytosis cannot explain the immunity which results from 

 the injection of filtered cultures, or of antitoxins, but when 

 blood serum of immunised animals was shown to possess antitoxic 

 properties, a new explanation of immunity was at once forthcoming. 

 In the light of these discoveries immunity, whether natural or 

 acquired, was regarded as due to the accumulation in the blood 

 and tissues of substances which have the property of counteracting 

 partially or entirely the products by which pathogenic bacteria 

 produce their poisonous effects. These antitoxins, or protecting 

 proteids, can be obtained not only from the blood but also from 

 the spleen and the lymphatic and other glands. They result 

 from the metabolism of the cells of the tissues of the body. 

 Phagocytes in their conflict with bacteria may play a small 

 part, but it is more than probable that immunity is altogether 

 independent of phagocytosis. 



