IMMUNITY IN TUBERCULOSIS 237 



and in some instances this arrest leads, through the changes induced 

 in the tuberculous tissue by means of the tuberculin injections, directly 

 to cure, or indirectly, through an increased power of resistance and 

 attack on the part of the forces of the organism, to eventual cure. But 

 a high and lasting degree of immunity has never been secured by the 

 use of tuberculin. This fact, disappointing as it was at first, is now 

 easily explicable. Tuberculin does not represent the entire series of 

 forces contained in the bacilli which the body has to resist in preserving 

 itself from infection with tubercular poison. The peculiar principles 

 contained in tuberculin are, indeed, not highly toxic for the normal 

 individual ; and our experience in securing immunity to micro-parasites 

 and their products has taught us that where no reaction or response to 

 the introduction of the foreign poison is called forth, no degree of pro- 

 tection to larger doses or more virulent poisons of the same nature 

 is to be expected. Toxic as is tuberculin to the tuberculous organism, 

 it is almost innocuous to the tubercle-free body. It has been found, in 

 keeping with this distinction, that the normal animal shows after 

 tuberculin treatment evidence of the minimal production of the neutral- 

 izing or antibody for the tuberculin, which, were tuberculin a direct 

 poison for the tissues, would probably be produced in larger amounts. 

 On account of this absence of action on the normal organism it has 

 been thought that the active principle in tuberculin does not exist in a 

 free state, but occurs in some combination, from which the tuberculous, 

 but not the non-tuberculous, organism can free it, and that the separa- 

 tion takes place in the tubercular foci upon which the specific action 

 of the poison is directly exerted. If this view is correct then the failure 

 of tuberculin to exercise any profound action on the healthy organism 

 is easily grasped. 



Increased knowledge of bacterial infection and immunity has taught 

 us that in case of bacteria which invade the depth of the body and pro- 

 duce their peculiar effects by reason of their immediate presence, we 

 can not expect to achieve marked immunity through the use of the 

 soluble gross-products of the parasites. The reaction of the body to the 

 invasion depends not upon the presence in the invader of one set of 

 toxic principles, but of many, some of which are contained in the solid 

 substance of the micro-parasite and do not go over into the fluids in 

 which they multiply. Thus it has been found, in case of certain bac- 

 teria, that a degree of immunity or protection which it is impossible 

 to obtain even after very prolonged treatment with the fluid portions 

 of cultures, can be secured quickly when small quantities of the living 

 or even dead microorganism are injected into the body. A high degree 

 of bacterial immunity has been secured up to now for a small number 

 of micro-organisms by vaccination — by the method introduced by 

 Pasteur — for several animal diseases, notably anthrax or splenic fever, 



