FACTS AND PROBLEMS OF IMMUNITY 317 



ture fluids or in the fluids of the animal body; that these bodies are not 

 readily, if at all, assimilable by non-phagocyting cells. These bodies 

 may, however, be broken up by digestive bodies present in the serum, 

 and from them may thus be liberated a poisonous substance, which may 

 then be assimilated by the higher cells of the body, and, when in suffi- 

 cient quantity, cause death. The more rapid the process of liberation 

 the more quickly death ensues. The plasma digestion is, then, according 

 to this conception, a mechanism which is faulty when applied to bacteria 

 and their products, and if this conception is correct the fault may occur 

 somewhat as follows: Bacteria and their insoluble or non-assimilable 

 products when taken into the phagocyte are subjected to two processes, 

 a primary bactericidal and coagulating one, and then a more leisurely 

 lytic or disintegrating action, during which poisonous products are 

 probably liberated, but slowly enough to be taken care of by destroy- 

 ing or neutralizing bodies. Even if the leucocyte dies, it is usually taken 

 up by a mononuclear cell, and the poisons do not become free in the 

 fluids. 



These are some of the problems of immunity, particularly those re- 

 lating to the microorganisms which are harmful to the animal body, 

 not so much through their ability to secrete harmful soluble poisons, as 

 through their insistently invasive character, or by the liberation of the 

 toxic products resulting from the destruction of their secretions or of 

 their own bodies. It is the diseases caused by these organisms on which 

 the attention of bacteriologists is now chiefly centered. 



The organisms of these diseases undoubtedly belong to two or more 

 classes, in one of which may be placed the typical septicemia producers 

 anthrax, pneumococcus, streptococcus, etc. in the other the less in- 

 vasive organisms, typified by cholera and to some degree by typhoid. 

 Between these two extremes there are all grades. 



If the data amassed in the study of these types of microorganisms, 

 and of the processes supposed to be involved in meeting infection and 

 establishing cure and immunity from them, have been maSe clear, it 

 may be easier to comprehend some of the problems which daily face 

 investigators in their struggle to arrive at a rational method of biologic 

 treatment, and to realize more fully, in the light of this knowledge, why 

 disappointment has so persistently followed in the wake of serum therapy 

 as applied to these infectious diseases. For, in spite of the most persist- 

 ent attempts to produce curative sera, the results have not been satis- 

 factory and have not led, except in rare instances, to the practical use of 

 such sera in the treatment of disease in man. 



