428 Major P. S. Leiean [April 23, 



The lethal power of bacterial emulsions, or of toxins, is determined 

 by finding the minimal measm'ed amount which, when injected, will 

 certainly kill an animal of a given weight in a given time. This 

 minimal lethal dose is known as the " M.L.D." Similarly, protective 

 fluids are standardized by ascertaining the minimal amount which, 

 when injected, will protect the animal against 100 " M.L.D.'s." 

 This amount is known as the " Immunity Unit." 



The affinities of these various substances for each other were 

 brilliantly explained and visualized by Ehrlich by means of his side- 

 chain theory, .which invokes the aid of imaginary " receptors " by 

 which the various bodies are attached to each other. This conception 

 will be illustrated presently by means of a working model, the move- 

 ments of which will be projected upon the screen. 



2. The toxins, or poisons (8 in diagram b), are formed by the 

 antigen and thrown off into the blood stream. They are ultra- 

 mici'oscopic, but so active that one fifteen-millionth part of a grain 

 is estimated to be sufficient to kill a rabbit. 



Being ultra-microscopic, they can in some cases be obtained in 

 the filtrate from which all bacteria have been removed. In other 

 cases the toxins, as those of typhoid bacilli for example, cannot be 

 obtained apart from the bodies of the bacilli. 



It is presumed that their action explains bcth chemiotaxis and 

 the production of anti-bodies. If highly concentrated they drive 

 away the phagocytes and kill the tissue-cells ; if dilute they attract 

 the phagocytes and stimulate the tissue-cells to manufacture pro- 

 tective substances. 



Their action is thus comparable to that of the fire of the host of 

 invaders, which drives the defenders out of their immediate area, 

 but stimulates defensive preparations throughout the rest of the 

 invaded state. 



3. Anti-toxins attach themselves to, and destroy, the toxins. 

 They are formed as a result of injection of living bacteria, dead 

 bacteria, or their toxins (6 in diagram c). It is they which confer 

 the passive immunity which follows injection of blood serum in which 

 they are formed as a result of inoculation of dead organisms into 

 living animals. By means of successive injections of a horse, e.g., 

 the resultant diphtheritic anti-toxins can be obtained so highly 

 concentrated that one dose of its serum contains sufficient anti-toxin 

 to neutralize 400,000 M.L.D.'s of toxin. 



The primary duty of the anti-toxins is to protect the tissue-cells 

 from toxins while the cells manufacture other anti-bodies which will 

 kill the invading bacteria, which the anti-toxins themselves cannot 

 attack. Although produced in excess, anti - toxins are rapidly 

 excreted, and it is to this fact that the evanescence of passive immu- 

 nity is due. 



Anti-toxins are thus comparable to the screen of artillery fire 

 which the defenders use to protect their arsenals, while the populace 



