IMMUNITY. GENERAL DISCUSSION 127 



opment of immunity. This theory has been tested in a very 

 great variety of ways, and seems to explain better than any other 

 most of the facts known relative to immunity. 



Duration of Immunity. Recovery from certain diseases, 

 such as small-pox, confers a lasting immunity ; from others, such as 

 pneumonia, a temporary immunity, and in still others the im- 

 munity disappears immediately upon recovery, as in influenza. 

 In some cases recovery from an attack seems actually to predis- 

 pose to a recurrence, as in erysipelas. This last probably means 

 simply the complete or relatively complete disappearance of im- 

 munity from a peculiarly susceptible individual. The duration 

 of immunity may depend in some cases upon the type of anti- 

 bodies produced, although this is certainly not always the case. 

 Probably the antibodies, particularly those introduced in passive 

 immunization, are eliminated in some of the secretions and ex- 

 cretions, or they may, in some cases, be easily destroyed. 



Antigens and Antibodies. It has been found experimentally 

 that injections of many substances besides bacteria and their 

 products cause reactions on the part of the tissues, with conse- 

 quent production of antibodies. These substances are called anti- 

 gens. An antigen is, therefore, that substance which, when intro- 

 duced into the body, stimulates the tissues to the production of 

 antibodies. An antibody may be more accurately defined as any 

 substance present in the serum which is capable of neutralizing, 

 antagonizing, precipitating, agglutinating, or dissolving the sub- 

 stance (antigen) which has induced the formation of such anti- 

 body. For example, the toxin of the diphtheria bacillus, when 

 injected in non-lethal doses, induces the production of antitoxin 

 by the tissues : the.fiwA8 is the antigen, the antitoxin, the anti- 

 body. Similarly, egg-white injected mto an animal would be 

 termed an antigen and the precipitating substance produced as 

 a consequence in the blood-serum is termed the antibody. 



Antibodies as Factors in Acquired Immunity. The somatic 

 reactions to the presence of antigens are now generally considered 

 of primary importance in acquired immunity. Certain bacterial 

 poisons called toxins cause the production by the animal tissues 

 of the antibody called antitoxin, which will neutralize the toxin. 

 The presence of certain bacteria in the body causes the production 



