NATURAL IMMUNITY 171 



body-cells, so that but a small amount of harmful substances are bound 

 to the body-cells, and no particular harm results, whereas a larger dose, 

 uniting with a greater number of cells, is capable of producing some dis- 

 turbance. 



6. A natural antitoxin immunity may exist, as the immunity of the 

 alligator to tetanus toxin, just mentioned. Similarly natural diph- 

 theria antitoxin may prevent infection, especially in those persons 

 known to harbor virulent bacilli in the fauces. In such instances, how- 

 ever, it is difficult to exclude entirely the possibility that a previous 

 minor infection has occurred, as natural antitoxin immunity persists 

 much longer than the passive immunity resulting from the administra- 

 tion of an antitoxin serum. 



Otto, who has recently investigated the content of diphtheria anti- 

 toxin in the blood of normal persons, found more than yj-g- of a unit of 

 antitoxin in each cubic centimeter of the blood of those who had been 

 in close contact with cases of diphtheria without having been sick them- 

 selves; others usually had much less. Observations would tend to 

 show that this quantity of antitoxin is generally sufficient to confer 

 immunity to diphtheria, and the object of von Behring's method of 

 active immunization is to induce the production of at least that much 

 antitoxin by the body itself. Otto found that diphtheria carriers, both 

 those who had had the disease and those who had not, contained more 

 antitoxin in their blood than* did patients who had just recovered from 

 an attack. This shows that the mere presence of bacilli in the throat 

 is sufficient to stimulate the production of antitoxin, on which the im- 

 munity of the carrier himself would seem to depend. Undoubtedly 

 physicians and nurses who are freely exposed to diphtheria and yet 

 escape infection owe their safety rather to an acquired immunity the 

 result of repeated contact with the bacilli, than to a natural antitoxin 

 immunity. 



7. In some instances a natural immunity may be dependent, at least 

 in part, upon antibacterial activity, due to the presence of bacteriolysins and 

 bacteriotropins in the body-fluids, as, for example, that shown by the dog 

 and the rat to anthrax. In other instances, however, a similar immunity 

 may be observed that cannot be ascribed to the presence of antitoxins or 

 bacteriolysins. In this type of immunity microparasites are apparently 

 unable to sustain themselves, and proliferate in one animal, although 

 aggressive enough in another of the same species. 



8. An immunity to infection, especially with such microorganisms as 

 the anthrax bacillus, which is markedly aggressive and but slightly toxic, 



