Biology of Man 519 



corpuscles in which the phagocytic cells engulf and destroy microorgan- 

 isms in the various body tissues, in the blood stream, in lymph nodes, and 

 in the phagocytic cells which line the capillaries of the liver and spleen. 



3. Defenses (Third line). — Other defensive reactions of the body 

 which depend upon previous or present contact with the infectious or- 

 ganisms are known as immu7iologic reactions. When infectious organ- 

 isms or their poisons penetrate to the deeper tissues, the body may be 

 stimulated to produce a series of substances which have the ability to 

 destroy or inactivate the organisms and to neutralize their toxic products. 

 These reactions are concerned with ridding the body of foreign proteins 

 which have been brought in by the invading organisms or through some 

 channel which is an unnatural method of entrance for that protein. 

 These reactions manifest themselves only under certain conditions and 

 only as a specific response to a specific protein. These reactions can, in 

 a general way, be illustrated by the introduction of egg proteins or simi- 

 lar foreign proteins. If egg protein is introduced into the digestive tract 

 it is digested and used in the natural building of protein substances in 

 the body. However, if this egg protein is injected directly into the blood 

 system, or in some other unnatural manner, the egg protein stimulates 

 the body to form a specific substance which will react specifically with 

 the egg protein. These specific substances appear in the blood stream 

 and are known as antibodies. The proteins which stimulate their for- 

 mation by the body are called antigens. For example, the foreign pro- 

 tein material of diphtheria toxin serves as the antigen to stimulate the 

 body to produce the specific antibody known as diphtheria antitoxin. 

 When the latter contacts diphtheria toxin, it neutralizes it, thus defend- 

 ing the body. 



Some of the more common antibodies may be described briefly as 

 follows : 



1. Antitoxins, which are substances which neutralize specific toxins 

 in an animal body or even in a test tube. They are formed only in an 

 animal body in direct response to toxins produced by bacteria, plants, 

 or animals; they are protein in nature and are aflPected by heat; they are 

 specific in their action; i.e., diphtheria antitoxin reacts only with diph- 

 theria toxin, and not with the toxins of tetanus, scarlet fever, gas gan- 

 grene, etc. 



2. Agglutinins, which are antibodies that agglutinate (clump) the 

 specific organisms which acted as antigens in their formation in a body. 

 The agglutination may occur in the body (hence, defend it) or outside 

 where it can be used for identifying specific organisms, for diagnosis of 



