CHAPTER XL 

 IMMUNITY AND ANAPHYLAXIS. 



Immunity, or resistance, may be denned as that power 

 or function of the living organism, natural or acquired, 

 which enables it to repel or prevent infection of itself by 

 micro-organisms or their products. 



Anaphylaxis, or excessive susceptibility (hypersuscep- 

 tibility, supersensitiveness), is defined as a state of 

 extreme sensitiveness to the injection of certain substances, 

 such as bacterial proteins, animal and vegetable albumins 

 (blood serum, egg white, milk), brought about by one 

 injection of the same substances or present from hereditary 

 transmission. 



Both these terms are relative, in most instances. Thus, 

 birds, while immune from tetanus toxin in any doses 

 likely to result from natural infection, may be killed by 

 enormous doses given experimentally. Similarly, in man, 

 the immunity conferred by one attack of a disease like 

 small-pox, may be overcome in special circumstances of 

 dosage and environment. 



Absolute immunity does exist. Thus, so far, no animal 

 has been infected with leprosy ; also, cold-blooded animals, 

 under their normal conditions, are absolutely immune to 

 the pathogenic bacteria of the warm-blooded animals. 

 The wild carnivora have a very high degree of resistance 

 to bacteria. 



Absolute anaphylaxis of a kind also occurs. Thus, the 

 injection (subcutaneously) of 0-25 c.c. of the serum of an 

 eel into a rabbit causes the death of the rabbit in a few 

 minutes. Also the offspring of animals which have been 

 themselves sensitized by injection, show a high degree of 

 anaphylaxis from birth. (The strict use of the term 

 " absolute " would require that the rabbit should die no 

 matter how small the dose of serum used. Thus its use 

 here is relative.) 



