378 BIOLOGY: GENERAL AND MEDICAL 



those substances are antigens that are capable of effect- 

 ing the essential physiologico-chemical reactions upon 

 which the antibody formation depends. The number 

 of antigens is, however, large, and they form a highly 

 miscellaneous collection, embracing bodies usually looked 

 upon as inert or inoffensive, bodies highly poisonous, 

 and even formed bodies, such as various cells red blood 

 corpuscles, tissue comminutions and even the micro- 

 parasites themselves. 



In all of the cases the antigen stimulates the forma- 

 tion of an antibody inimical to itself. If it be a toxin, 

 to the formation of a neutralizing body or antitoxin; if 

 it be enzyme, to the formation of an antienzyme; if a 

 cell or formed body, to the formation of a cytotoxin or 

 cell-dissolving body. Thus the antibody is always 

 specific; i.e., reactive upon its antigen alone. 



The presence of the respective antibody in its blood 

 confers increased resisting powers upon the animal in 

 whose blood it is, hence its acquired immunity. In 

 rare cases the immunity disappears and the animal 

 develops a mysterious hypersensitivity not accounted 

 for by anaphylaxis as ordinarily understood. 



By long-continued immunization of large animals to 

 certain antigens, such as diphtheria toxin, tetanus toxin, 

 venom, etc., we may be able to secure sufficient quanti- 

 ties of antibodies antitoxins to be made practical 

 use of for the treatment of the respective intoxications 

 diphtheria, tetanus, and snake bite. 



The reaction between antigen and antibody is chemical 

 in nature, but varies in quality. The reaction between 

 toxin and antitoxin is direct and immediate; that be- 

 tween the antigens and the various cytotoxins indirect 

 or intermediate i.e., taking place only in the presence 

 of a third substance. 



Such indirect chemical actions are in perfect accord 

 with other physiological processes, and the reader will 

 recall that pepsin can only act upon proteins in the 

 presence of HC1; trypsin upon proteins only in the pres- 



