698 MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMALS 



Passive Immunity. Passive immunity may be conferred by the 

 injection of antitoxins, and by the injection of bactericidal substances. 

 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 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 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. In the 

 same way tissue cells cannot utilize certain foods because they have no 

 combining groups. If all the tissue cells in the body are in this condi- 

 tion 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 sus- 

 ceptible. 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 (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, will remain in the circu- 

 lation 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 in- 

 jected. Some antigens have special affinities for certain tissues, as 

 for example, tetanus toxin and nerve cells. In this case, however, the 



