74 ESSENTIALS OF BACTERIOLOGY. 



blood-serum of immune animals is simply another means for 

 immunization. It is less permanent than the other forms of 

 immunization, but it appears very soon after the injection, and 

 in a modified form has a curative action even when the symp- 

 toms of the infection are already present in the system. 



Inherited Immunity. An immunity to disease acquired dur- 

 ing the lifetime of the parents is probably never transmitted 

 to the offspring, though the mother may transmit a temporary 

 immunity to the child in utero or the child itself may have 

 been subjected to the infection at the same time with its 

 mother. But this cannot be called inherited. 



Theories of Immunity. 



Several older theories need only to be mentioned, as they are 

 no longer tenable. They are the exhaustion theory of Pasteur, 

 the retention theory, and the humeral theory. At present 

 modifications of MetschnikorF's phagocytic theory and Ehrlich's 

 side-chain theory seem the most plausible. 



Phagocytic or Cellular Theory. Metschnikoff elaborated this 

 after his study on inflammation. Phagocytosis occurs in 

 animals when subjected to the action of an irritant. The leu- 

 cocytes are attracted to the injured spot and envelop the irri- 

 tating substance, be it bacteria or dead matter. The theory 

 given out at first was that if the leucocytes conquer the bacteria, 

 immunity results; if the bacteria eat up the leucocytes, disease 

 occurs. 



Modified to suit other conditions, as, for instance, the germi- 

 cidal properties of serum freed from its cellular elements, 

 Metschnikoffnow states that at times phagolysis that is, breaking 

 up or solution of the phagocytes takes place, and the fluids in 

 which these cells are dissolved become charged with the powers 

 originally present in the phagocytes. Chemotaxis is the term 

 applied to the attraction of bacteria for the leucocytes, and is 

 supposed to be chemical in its nature. The phagocytic cells 

 comprise : () The polymorphonuclear leucocytes of the blood, 

 termed microphages, and (6) a group called macrophages which 

 includes all other cells having phagocytic properties, such as 

 leucocytes other than the polymorphonuclears, endothelial 

 cells, and connective-tissue corpuscles. When these cells are 



