IMMUNITY AND SUSCEPTIBILITY. 553 



not react to any great extent and the injected antibodies remain. Various 

 other antibodies may be injected into other animals and confer upon 

 them passive immunity. 



The principal antibodies produced in active immunity will be subse- 

 quently discussed. 



THE ORIGIN AND OCCURRENCE OF ANTIBODIES. 



The toxic and some of the non-toxic substances of bacteria and cells 

 from other sources when introduced into the body of a susceptible animal 

 usually have the power to produce antibodies. Substances having the 

 power of producing antibodies are known as antigens. Among the 

 antibodies produced are antitoxins, bactericidal and lytic substances, 

 opsonins, antiferments, agglutinins, and precipitins. The antigenic 

 substances for these antibodies will be discussed later. The mechanism 

 of action of the antigen is of interest. It is supposed that the antigen 

 can combine only with the cell which has the proper combining groups 

 or receptors. The antigen combines in the same way that food products 

 combine with the tissue cells. In case there is no group in the tissue 

 cell with which the antigen can combine, that tissue is naturally immune 

 to the antigenous substances in question. If all the tissue cells in the 

 body are in this condition then the individual may be said to be naturally 

 immune. It occasionally occurs that certain cells of the body are not 

 susceptible to the action of antigens at one time, while at another they 

 are susceptible. For example, the red blood corpuscles of the young 

 chick are not affected by the lysin in spider poison, while those of the 

 adult are readily hemolyzed (hemoglobin liberated) . It also occurs in rare 

 cases that the antigen, when injected into an animal whose tissue cells 

 show no affinity for it or no proper receptors, that it will remain in the 

 circulation for days and weeks without combining and producing any 

 effect. The antigen, for example a toxin, can be isolated from the blood 

 in such a case in the same concentration and form as when it was injected. 

 Some antigens have marked affinities for certain tissues, as for example, 

 tetanus toxin and nerve cells. In this case, however, the large part of 

 the antitoxin is produced by cells other than those of the nervous system. 

 The production of antibodies for antigens probably occurs in the follow- 

 ing way: the antigenous substances combine with the cells, utilizing 

 all the available receptors, leaving none open for food, and thus pervert- 



