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 protection 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 neutralizing 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 separation 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 produce 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 bacteria, 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,
