FACTS AND PROBLEMS OF IMMUNITY 317 



body are secreted or given off from the cell. Such lytic amboceptors, 

 then, when present in the plasma and activated by complement, may 

 thus become an active agent for harm by liberating poisonous sub- 

 stances from the bodies of germs which are susceptible to such action, 

 or from the insoluble or non-assimilable products of these or more lysis- 

 resisting members of the invasive organisms ; and by the action of these 

 poisons, phagocytosis may be hindered and the specialized cells poisoned. 

 Since the neutralizing or poison-destroying bodies are not present in the 

 plasma, the leucocytes are then poisoned from without, just as are the 

 specialized cells, and the more active the plasma digestion, the more 

 deranged the true protective mechanism becomes. 



These are some of the problems of immunity, particularly those 

 relating 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 made 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. 



The sera, thus produced, have not, except in a very minor way, 

 been antitoxic in the usually accepted sense, and depend, as we have 

 seen, probably, for any protective value they may possess, on their germi- 

 cidal and bacteriolytic power and on the opsonins they may carry, and 

 thus facilitate phagocytosis. These sera are capable of protecting an 

 animal from an infecting organism, when mixed with it in surprisingly 



