114 PRINCIPLES OF PHYSIOLOGY 



body, cause disease, either by multiplying in 

 enormous numbers or by producing substances, 

 called toxins, which act as poisons. A toxin 

 is probably of the nature of a protein or 

 proteose, and usually there is associated with 

 it another body, called an antitoxin, sad to 

 be of the nature of a globulin. Toxin and 

 antitoxin neutralize each other, so that a 

 mixture injected into an animal may produce 

 no effect. By a system of inoculating a 

 healthy horse with small but increasing doses 

 of the diphtheria virus, the serum of the 

 animal by and by contains a large amount of 

 antitoxin, and the injection of this serum in a 

 case of diphtheria may save life by neutralizing 

 the toxin of the disease. Sera prepared in this 

 way are now used in practical medicine with 

 beneficial effects. Further, they may confer 

 immunity, that is to say, the injection of such 

 fluids may protect against attacks of the 

 disease. Thus the body may be protected 

 against the invasion of specific organisms by 

 the phagocytic action of leucocytes, by the 

 globulicidal action of various substances in 

 the blood, and by the formation of antitoxin. 



