[nicholls] unsolved PROBLEMS OF IMMUNITY 281 



sheep, or by the 23ro<iucts of their growth, the bacteria having been 

 filtered off or destroyed by heat, as in Hafl'kine's method of immuniz- 

 ation against plague, and AYright's antithyphoid inoculations. Such 

 methods, however, are in general more efficacious in preventing infec- 

 tious disease than in curing it when once it has become established. 

 A notable exception to this rule is met with in the case of diphtheria, 

 in which disease we have in an antitoxic serum a powerful therapeutic 

 agent. This, however, is prepared in another way. An animal, in 

 this case a horse, is rendered immune by one or other of the methods 

 mentioned, and when liighly refractory is bled. The serum is separated 

 from the blood and a certain amount of this when injected into a 

 second individual sufFering from diphtheria Avill often suffice to bring 

 about cure, or, again, protect against the disease in those exposed to 

 mfection. This form of immunity has been termed by Ehrlich passive 

 itnmunifij. It is temporary in character and is brought about by the 

 action of anti-bodies, elaborated in one individual, which neutralize 

 the effects of bacteria which have gained an entrance into a second. 

 With regard to the method l^y which the economy protects itself 

 against the onslaught? of the infecting micro-organisms many different 

 opinions have been expressed. Klebs and Pasteur held that during 

 the 'first attack of any given disease the germ used up all the available 

 pabulum and that its existence came to an end, as it were by starv- 

 ation. U]itil this food-stuff was recreated the person was immune. 

 Chauveau was of the opinion that, the toxic emanations from the bac- 

 teria kept on accumulating in the system until they reached such a 

 degree of concentration that farther existence of the germ became impos- 

 sible. It was, in other words, poisoned by its own excreta. Grawitz 

 explained the production of immunity on the theory that infection is 

 H combat between the germs and the cells of the body. If the cells 

 are victorious in the conflict they acquire increased powers of resistance 

 to the germ in question and subsequently are able to destroy it as 

 soon as it gains an entrance into the tissues. These various ideas 

 need only be mentioned, for they are not seriously entertained by path- 

 ologists at the present day. Two chief opposing schools of thought 

 only may be said to be in existence now. Metschnikoff, the apostle 

 of the doctrine of phagocytosis, would attribute the cure of infective 

 disease and the production of immunity to the activities of special 

 cells — phagocytes — especially certain leucocytes of the blood, the 

 splenic corpuscles, the lining cells of blood- and lymph-channels and 

 of serous sacs. The phenomena of phagocytosis are directly dependent 

 u])on the attraction (positive chemotaxis) existing between these cells 

 and the bacteria and bacterial products. By their amoeboid powers 



