36 ESSENTIALS OF BACTERIOLOGY 



(F) Inoculation with sublethal doses of virulent bacteria, 

 beginning with small doses, and gradually increasing their 

 size. Guinea-pigs inoculated in this way have acquired a 

 marked degree of immunity to tuberculosis. 



Passive acquired immunity is always artificially sup- 

 plied to the animal. It follows when antibodies are supplied 

 from an immunized animal to one normally susceptible. 

 Immunization against diphtheria by the injection of diph- 

 theria antitoxin is a good example. 



Theories of Immunity. Phagocytic Theory of Metchni- 

 kojff. Immunity is dependent on the action of the phago- 

 cytes and their ferments. The phagocytes are of two kinds 

 macrophages, which include endothelia and connective-tis- 

 sue cells, and microphages, the polymorphonuclear leukocytes. 

 These phagocytes liberate ferments macrocytase and micro- 

 cytase respectively. Infecting organisms and their toxins 

 are destroyed by the phagocytes and their ferments. This 

 theory has been replaced by the lateral or side-chain theory of 

 Ehrlich. 



Ehrlich's Lateral Chain Theory. This derives its name 

 from the fact that it presents an analogy to what happens 

 in the benzol ring of organic chemistry when its replaceable 

 atoms of hydrogen are substituted by "side chains" of more 

 or less complex nature. The molecule of protoplasm is sup- 

 posed to consist of a central atom group, provided with a 

 large number of side chains which subserve the vital processes 

 of the molecule by combining with other organic molecules. 

 These side chains are called receptors, and are of many dif- 

 ferent kinds, so as to fit them for combination with many 

 different varieties of extraneous groups. 



Three orders of receptors are described: Receptors of the first 

 order, which concern themselves with the assimilation of 

 simple substances (toxins, ferments, and other cell secre- 

 tions), utilizing a single haptophore. Antitoxins, as an 

 example. 



Receptors of the second order, which, in addition to the 

 haptophore group, possess a second group, which affects the 



