ACTIVE AND PASSIVE IMMUNITY 



I6 3 



Not only may bacteria act in this way but foreign cells, such as red cells or various 

 parenchymatous cells, when injected, give^rise to antagnnisHr substances which act 

 as factors in their destruction haemolysins for red cells, cytolysins jor different 

 parenchymatous cells. Such methods produce " active immunity." 



The substance which is injected and in reaction to which antibodies 

 are produced is called a,n]antigen. 



2. When we take the serum of a man or animal immunized actively and inject 

 it with its contained antibodies into a second animal or man, we confer an immunity 

 on the second animal; but as his cells take no active part in the production of the 

 immunity, but are only passive, we term this immunity "passive immunity." If 

 this serum which is introduced in passive immunity only neutralizes the toxic prod- 



FIG. 46. Receptors of the second order and of some substance uniting with one 

 of them. (Journal of the American Medical Association, 1905, p. 1131.) c, Cell 

 receptor of the second order; d, toxophore or zymophore group of the receptor; e, 

 haptophore of the receptor; /, food substance or product of bacterial disintegration 

 uniting with the haptophore of the cell receptor. 



ucts of the infecting bacteria, we term it antitoxic passive immunity and designate 

 the immune serum as antitoxic serum. If it destroys the organism, we call it_anti- 

 microbic serum, and the immunity, antimicrobic passive immunity. Some immune 

 sera are both antitoxic and antimicrobic. 



Toxins. It is well to remember that some organisms produce ajsoluble or^extra- 

 cellular toxin which is given off while the bacterium is alive: and in other instances 

 the toxin is Jntracellular and is only given off when the bacterium disintegrates: 

 consequently, an antimicrobic serum may cause the liberation of toxin. Diphtheria, 

 tetanus, or b^tujjism antisera are instances of antitoxic sera, while practically all others 

 a7e""antimicrobic. The antidysentery serum against Shiga strains seems to have 

 antitoxic power. B. pyocyaneus also has a soluble toxin. There is but one factor to 



