HYPERSENSITIVENESS 509 



These earlier results of Seibert have been reinvestigated by 

 Seibert, Pedersen and Tiselius (1938) by means of the ultra- 

 centrifuge and cleetrophoresis. They have obtained from TPA 

 an antigenic protein with a molecular weight of 32,000 and from 

 PPD a protein with a molecular Aveight of 17,000 to 18,000. These 

 two proteins would elicit both local and systemic reactions in 

 tuberculous guinea pigs. Positive skin reactions were obtained 

 with substances of smaller molecular weight obtained from old 

 tuberculin. 



Seibert (1941) has since modified the method of PPD production 

 by carrying out the procedure at 4° to 5° C, using less heat and 

 weaker acid solution and drying the final product by the lyophile 

 process. The i)roduct obtained by this pi'ocedure is twice as potent 

 biologically and much purer than pi-evious preparations. Th(! 

 surprising thing is that while the molecular weight is only al)ou1 

 10,500 this more i)otent tuberculin is more antigenic than the 

 previous PPD protein preparations, having a molecular weight of 

 17,000 to 18,000. This has led Seil)ert to think that the size of the 

 molecule is not so important in antigenicity as she previously 

 tliought. She is now of the opinion that the potency is inherent 

 in one part of the protein molecule while the antigenicity is de- 

 pendent upon another part of the same molecule. 



The advantage of the purified protein derivative of tuberculin 

 (PPD) lies in the fact that it can be injected repeatedly into the 

 same person without causing skin allergy to develop. The method 

 of preparation of the new PPD is given by Seibert and Glenn 

 (1941). Just why some of these highly purified fractions give a 

 higher percentage of reactors in the second injection than previous 

 preparations Ls unknown. Seibert (1941) says that this point is 

 being investigated. 



Chemicai> Nature of PPD. — Seibert (1941) says that the new, 

 (.Iried PPD is almost colorless, quite soluble in water, and contains 

 approximately 1.2 per cent nucleic acid and 5.9 per cent polysac- 

 charide. Its potency is twice that of the previous PPD. This is 

 indicated by the fact that 0.00001 mg. produces as strong reactions 

 in sensitive patients as 0.00002 mg. of the previous product. 



According to Chase and Landsteiner (1939) the separation of 

 tuberculin into two fractions, one responsible for skin reactions 

 and the other for systemic reactions, is reported by ]\Iaschmann 



