ANTI-TOXINS 65 



followed by an immediate fall in the anti-toxic power of the serum, 

 only to be followed by a quick rise. The horse will not produce 

 anti-toxin indefinitely. After the animal has been immunized suf- 

 ficiently, his blood is drawn from the jugular vein, and after the clot 

 has formed the serum is drawn off and stored. 



McFarland found that a horse was capable of producing enough 

 anti-toxin to protect 806 other horses against doses of toxin, each one 

 of which was equivalent to the total amount of toxin that the immun- 

 ized horse received. Thus there is evidently a tremendous over- 

 production of anti-toxin far above the needs of the animal. 



The various component parts of the toxin stimulate the cells of 

 the horse to produce the receptors, or anti-toxin. The toxoids, 

 themselves not poisonous, have the property of stimulating the pro- 

 duction of anti-toxin. We measure the anti-toxic powers of the anti- 

 toxin with units arbitrarily devised. An anti-toxic unit- is ten times 

 the least amount of anti-toxic serum that will protect a guinea pig weigh- 

 ing 300 grams (standard) against ten times the least certainly fatal 

 dose of diphtheria toxin. 



To standardize anti-toxin, we must employ animals, into the 

 bodies of which toxins and anti-toxins are injected. If a certain 

 amount of anti-toxin is necessary to protect a guinea pig against ten 

 times the minimum fatal dose of toxin per 100 grams of guinea pig 

 weight, then we know that the anti-toxin contains so many units. A 

 unit dose of toxin is the smallest amount of toxin necessary to kill a 

 guinea pig weighing 300 grams, or the dose per 100 grams of guinea pig 

 necessary to kill. 



Ehrlich's method of standardizing is to obtain an anti-toxin of 

 known strength (anti-toxins do not deteriorate or vary as do toxins) . 

 A standard anti-toxin made by Ehrlich is now everywhere used, and 

 is furnished by him from his institute. 



Against this standard anti-toxin a toxin of unknown strength is 

 measured by means of guinea pigs. The toxin unit thus founcl is 

 then used to determine the anti-toxic unit of anti-toxins of unknown 

 power. 

 5 



