698 MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMALS 



In this type of artificially produced acquired immunity the body cells 

 do not react to any great extent and the 'injected antibodies remain 

 practically unaltered. Various other antibodies may be injected into 

 other animals and confer upon them passive immunity. 



The principal antibodies produced in active immunity will be 

 subsequently discussed. 



The Origin and OccimRENCE or 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 com- 

 bining 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-toxin 

 in spider poison while those of the adult are readily hemolyzed (hemo- 

 globin 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, will remain in the circulation for days and weeks 

 without combining and producing any effect. The antigen, for ex- 

 ample, 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 special affinities for certain tissues, as for example, tetanus toxin 

 and nerve ceUs. In this case, however, the larger part of the anti- 

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

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



