136 INFECTION AND IMMUNITY. 



that it probably does not include all conceivable 

 factors. 



FACTORS IN IMMUNITY. 

 NATURAL IMMUNITY : 



1. Antibacterial properties of the serum or plasma. 



2. Antibacterial properties of the leucocytes in cooperation 



with opsonins (preparers for phagocytosis). 



3. Antitoxic properties of the serum or plasma, resulting 



in a simple binding or neutralization of toxins. 

 Hypothetically, too, certain ferments of the body 

 may split or decompose toxins. 



4. Total insusceptibility of the body to endotoxins or ecto- 



toxins. There is reason to believe also that com- 

 parative resistance sometimes depends on the fact 

 that a toxin has a stronger affinity for organs of less 

 vital importance for the life of the individual than it 

 has for more important organs. The chicken, for 

 example, is very resistant to tetanus, although its 

 serum contains no antitoxin. It yields, however, 

 when inoculations are made directly into the nervous 

 tissue. 



5. The "atreptic" immunity of Ehrlich, in which micro- 



organisms do not find suitable food, or food in avail- 

 able form, in the body of the host. This is theoreti- 

 cal at present. 



ACQUIRED IMMUNITY : 



1. Increased antibacterial properties of the serum or 



plasma. 



2. Increased phagocytic power of the leucocytes, and per- 



haps other phagocytic cells, depending on the increased 

 formation of opsonins. 



3. Increased antitoxic properties of the serum or plasma. 



4. Habituation of the cells to bacterial poisons. 



Under 3 and 4, we may have also to consider an 

 increased power of splitting or digesting toxic sub- 

 stances. Of this, however, we know nothing defi- 

 nite. 



One and 3 (possibly also 2) may be passive as 

 well as active. These subjects receive further con- 

 sideration in subsequent chapters. 



