TYPES OF IMMUNITY. 



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mune serum (such as diphtheria antitoxin) is a 

 passive immunity, since it depends on the intro- 

 duction of ready-made immunizing substances 

 rather than on their production through an active 

 process on the part of the one injected. Active 

 and passive immunity, then, are varieties of ac- 

 quired immunity. Depending on the disease 

 which caused the immunity, or on the character 

 of the serum injected, they may be antibacterial 

 or antitoxic, or, to a certain extent, opsonic (pha- 

 gocytic). 



Any one of the types mentioned may be either 

 relative (partial) or absolute (complete). If the 

 immunity is absolute, infection is impossible. If 

 only relative, different conditions may be made 

 to prevail which would render infection possible; 

 for example, a large number of bacteria will often 

 cause an infection where a smaller number fails 

 to do so. There may also be a temporary decrease 

 in one's resistance through overwork, hunger or ex- 

 posure. Immunity is usually relative. 



By proper combinations of the terms which 

 have been enumerated, one may describe somewhat 

 accurately the different forms of immunity. Thus, 

 a child which has received a prophylactic injec- 

 tion of diphtheria antitoxin is in a state of ac- 

 quired passive antitoxic immunity to diphtheria. 

 If immunity to typhoid has developed as a result 

 of the disease, the condition is that of an acquired 

 active antibacterial immunity, etc. Accordingly, 

 although tKe terms may be somewhat confusing, 

 it is seen that they are in no sense contradictory. 



The following classification of the agencies which 

 may contribute to immunity in one disease or 

 another may be given, although we must recognize 



Relative and 



Absolute 



Immunity. 



Classification 

 of Types. 



