114 ESSENTIALS OF BACTERIOLOGY. 



had a principle in their blood that was antagonistic to the 

 development of the toxin. 



Immunity. Brieger and Frankel, by injecting 10 to 20 c.cm. 

 of a three weeks' old culture of diphtheria bacilli, which had 

 been heated at 70 C. for one hour, produced an immunity in 

 guinea-pigs against the virulent form. 



This important discovery has been utilized in a practical way. 

 Horses are made immune by gradually increased doses of the 

 toxin until 300 c. cm. can be borne without bad effect. This 

 may require several months' time. The serum of such an im- 

 munized animal is now possessed of antitoxic properties. 



Behring has standardized the strength of antitoxic serum, so 

 that we say a serum has an immunizing strength of 60 units 

 or 100 units, which means that 0.1 c. cm. of the serum would 

 protect against 1 c. cm. of the toxin when injected together into 

 guinea-pigs. 1 cubic centimetre of this is the unit. The strength 

 commonly employed in human beings is 1500 units in 10 c. cm. 

 If this amount is injected into a child suffering from diphtheria 

 in the earlier stages (second to third day), the disease is often 

 arrested. The membrane begins to disappear, and in two or 

 three days has vanished. The constitutional symptoms are 

 likewise greatly influenced by the injection. If a smaller dose 

 is injected into persons who have been exposed to contagion, 

 the disease is prevented from appearing. 



The antitoxin has no influence on the bacteria themselves; 

 their virulence and length of residence in the body is not 

 lessened. 



The toxin generated by the germ is supposed to be neutral- 

 ized by the antitoxin and prevented from injuring the body 

 tissues. 



Pseudo-diphtheria bacilli, so called, differ from the true 

 organism in certain cultural and morphological characteristics, 

 and do not produce a toxin, but their true status is still uncer- 

 tain. 



Site of Bacilli. Bacilli are usually found in the older portions 

 of the pseudo-membrane very near to the surface. The secre- 

 tions of the throat of a diphtheritic child produced bacilli three 

 weeks after the temperature was down to normal. 



