Immunity m Pneumococcal Infections. 



Congenital immunity or unsusceptibility towards pneumococcal infec- 

 tion is possessed in different degrees by animals of different species, 

 pigeons and domestic fowls being highly immune, the human species, 

 guinea-pigs, rats, dogs and sheep being more susceptible, while rabbits 

 and mice fall an easy prey to pneumococcal disease. Acquired immunity 

 is that attained naturally, by passing through an attack of pneumococcal 

 disease, or artificially, by the inoculation of the microbe itself or its 

 products — active immunisation — or of the antibodies produced in the 

 actively immunised organism — passive immunisation. But on the 

 quantity and quality of the immunity, and on the mechanism by which 

 immunity is conferred, unanimity of opinion has not been attained, 

 though investigated from many directions and by widely varying 

 methods. 



Acquired Immunity. — The immunity naturally acquired by passing 

 through an attack of the chief pneumococcal infection — pneumonia — 

 does not appear to be very durable. Thus Rieselle {}) in lOO cases of 

 pneumonia found that only 50 had it for the first time, that 32 had it for 

 the second time, and the rest for the third time or oftener. Mollmann 

 (2) in 832 cases noted that only 13 per cent, were in their first attack of 

 pneumonia. In 1800 cases collected from the literature on this subject 

 420 had previous attacks of pneumonia {i.e., 23*3 per cent). The 

 frequent occurrence, however, of empyaema, meningitis, arthritis, endo- 

 carditis, etc., following pneumonia, would suggest that no immediate 

 immunity is conferred. Yet, in the majority of cases, individuals who 

 have passed through an attack of pneumonia enjoy immunity for a short 

 time at least, and animals which have been inoculated with pneumococcus 

 cultures, and which have recovered, are more or less resistant for some 

 time. 



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