IMMUNITY IN DIPHTHERIA. 403 



ity. The prolonged presence of bacilli of low 

 virulence in the nose or pharynx, or mild attacks 

 of the disease which have not been recognized, 

 may cause the development of antitoxin. As stat- 

 ed in an earlier chapter, the loss of suitable re- 

 ceptors may be a factor in this type of acquired 

 immunity. 



Hypertrophic tonsils and chronic pharyngitis 

 appear to be predisposing causes in children. 



Spontaneous recovery (active immunity) is due Active 

 to the formation of the specific antitoxin by 

 the tissues of the patient. We may regard the 

 relationship of the leucocytes to diphtheritic in- 

 fection as not definitely settled. Although leuco- 

 cytosis is a fairly constant occurrence and may go 

 as high as 25,000 to 30,000 to the cubic milli- 

 meter, it is difficult to dissociate that due to the 

 diphtheritic infection from that caused by a mixed 

 infection with the streptococcus. Both polynu- 

 clears and mononuclears are increased, the latter 

 being especially marked in children (Ewing). 

 The opsonin content of the serum in diphtheria is 

 below normal at the onset of the disease. As the 

 symptoms subside and the membrane disappears, 

 the opsonic index rises considerably, returning to 

 normal in from two to nine days. 



Injection of dead diphtheria bacilli in suitable 

 numbers into rabbits is followed by a rise in the 

 opsonic index. Injection of dead diphtheria bacilli 

 may prove of service in ridding the throats of 

 bacillus-carriers of bacilli (Tunnicliif). 



Eecent experiments have substantiated the ideas 

 of Behring that bacteriolysins are of little impor- 

 tance in immunity in diphtheria. 



