66 . IMMUNITY 



The power of anti-toxic sera varies; some contain from 200 to 300 

 units only per c.c.; others may contain even 1,700 or 2,000 per c.c. 



An ti- toxic serum is preserved by the addition of .5 percent of 

 tri-cresol or phenol. It remains practically unchanged in strength 

 for a year or more. When used it is common to inject from 2,000 

 to 5,000 units. 



It is not only of value as a curative agent, neutralizing the toxins 

 already formed, but is valuable as an immunizing one against infec- 

 tion. If injected early in a case of diphtheria, it is much more likely 

 to do good, than if used later. Some desperate cases have received 

 100,000 units and have recovered. 



Tetanus Anti-toxin. Tetanus anti-toxin is produced in a man- 

 ner similar to that of diphtheria anti-toxin. As the horse is exceed- 

 ingly sensitive to tetanus toxin, before the immunizing process is 

 begun, the toxin is attenuated by heat or iodine. 



The anti- toxin is standardized, as in diphtheria, by testing its 

 potency against the toxin. A guinea pig of 500 grams weight is 

 used, and test toxin is employed of such strength that .01 c.c. will 

 kill this guinea pig in about four days. This amount of toxin is 

 neutralized by ygV^r f a un ^ f anti-tpxin, or one unit of anti- 

 toxin will protect 1,000 guinea pigs against the minimum fatal dose 

 of tetanus toxin. The United States unit of tetanus anti-toxin is 

 now the least quantity of anti-tetanic serum necessary to save the 

 life of a 350 gram guinea pig for ninety-six hours against the official 

 test dose of standard toxin furnished by the Hygienic Laboratory 

 of the Public Health Service. 



There can be no doubt that tetanus anti-toxin, if given with the 

 toxin or soon afterwards, is a potent means of preventing lethal 

 action of the toxin. Tetanus toxin enters into such quick combina- 

 tion with the cells of the motor elements of the nervous system, and 

 the union is so permanent that it is difficult for the anti-toxin to form 

 any union with the combined toxin. If an immune animal whose 

 blood is powerfully anti-toxic received into his central nervous sys- 

 tem a dose of toxin he will succumb at once; the anti- toxin appar- 



