CHAPTER IX 



THE VARIOUS TYPES OF IMMUNITY 



As has been stated in the preceding chapter, it is generally agreed 

 that various antibodies and other protective agencies exist, although 

 opinions differ as to the source and relative importance of these to re- 

 sistance to and recovery from various infections. Whether or not a 

 particular antibody is derived from a certain group of cells is largely a 

 matter of individual opinion, because of the difficulty of deciding the 

 point by actual experimental evidence. Of far more importance is a 

 knowledge of the properties of antibodies and of the role they may play 

 in the processes of immunity. It is seldom that resistance to, or re- 

 covery from, an infection is dependent upon one defensive factor: 

 usually several agencies are operative, although one factor may pre- 

 dominate. For example, antitoxins are known to neutralize their 

 respective toxins, and are of most value in combating the toxemias, 

 such as diphtheria and tetanus; bacteriolysins cause the death of and 

 may totally destroy their antigens, and play an important part in the 

 recovery from infections with bacilli of the typhoid-colon and cholera 

 groups; phagocytosis in itself is of importance in staphylococcus in- 

 fections, and is of primary importance, in conjunction with the opsonins, 

 in recovery from pyogenic infections in general; agglutinins and pre- 

 cipitins do not appear to have a direct inimical influence on their 

 antigens, but are probably secondary factors, and contribute in some 

 manner toward their destruction. Along with important non-specific 

 factors, these various antibodies are responsible for the different forms 

 of immunity, which may now be considered in their more general as- 

 pects. 



There are two forms of immunity natural and acquired. 



NATURAL IMMUNITY 



Natural immunity is the resistance to infection normally possessed, 

 usually as the result of inheritance, by certain individuals or species under 

 natural conditions. 



The mechanism of this type of immunity is very complex, and bears 

 an intimate relation to the subject of infection, both local and general, 



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