102 THE INDIVIDUAL ORGANISM 



lar antigen, and is fully effective only against that antigen, though it 

 may react to some extent with chemically related substances. The causa- 

 tive agents of diphtheria, smallpox, measles, typhus, yellow fever, and 

 other diseases stimulate the production of antibodies which are highly 

 specific and in most cases give partial or complete immunity against 

 subsequent attacks. The action of the blood antibodies supplements that 

 of the antibodies present in the invaded tissues and becomes of utmost 

 importance in those instances where microorganisms or antigens gain 

 access to the blood itself. 



In present-day medical practice two main types of immunity are recog- 

 nized — active immunity, developed by the body in response to entry of 

 antigens or antigen-containing microorganisms, and passive immunity, 

 produced by injecting into the body a serum containing antibodies or 

 antitoxins actively produced by some laboratory animal. Active im- 

 munity is comparatively lasting and in many instances permanent; 

 passive immunity endures only so long as the foreign antibodies or anti- 

 toxins persist in the person receiving them. 



The common phenomenon of allergy is an aspect of tissue immunity 

 that deserves brief mention. Many persons become "sensitized" to 

 specific antigens to such a degree that exposure to minute amounts of 

 these substances produces disproportionately large reactions. The symp- 

 toms are varied and may include hives, skin rash or inflammation, eczema, 

 and asthma. If the antigen is injected into the blood stream, death from 

 immunologic shock may follow. The causes of allergy are still obscure, but 

 we may quote Kahn on this point. He says: "Neither immunity nor 

 allergy can be fully understood without realizing the tremendous burden 

 of defense carried on by the exposed tissues of the body. Through evolu- 

 tionary ages these tissues had the task of warding off microorganisms, 

 and preventing their entrance into the blood stream and into the deeper 

 tissues. Is it not significant then that these surface tissues should be 

 largely involved in allergic disturbances of man? . . . Like over- vigilant 

 guards, the surface cells begin to treat harmless substances as though they 

 were harmful microorganisms, and react with great intensity on slight 

 provocation. ... It would appear that the allergic person suffers from 

 •hyperactivity of the immunologic function." 



