42 JOHN H. NORTHROP 



curves have long "lag periods" and are not strictly logarithmic except 

 for a small part of the reaction. They agree just as well or better with 

 the integral of the usual probability curves. (For the mathematical 

 relation between logarithmic and probability curves, see Yule, 1910.) 

 The case is very similar to the rate of death curves of bacterial cultures 

 or of insects (Loeb and Northrop, 191 7). The data may be made to fit 

 a log curve if the "lag period" is neglected. The appearance of antibody 

 in the blood, plotted as log curves (Burnet, Freeman, Jackson, and Lush, 

 1941), and as probability curves, is shown in Fig. 3. From the point 

 of view of probability curves, the results indicate that the cells in which 

 the antibody is produced die or are ruptured when antibody is released 

 into the blood stream. The rapid and large rise sometimes noted on a 

 second injection of antigen may thus be related to the phenomenon of 

 anaphylaxis. The cells are sensitized by the first small injection and 

 injured by the second. This mechanism agrees with the process of 

 antibody formation suggested by Sabin (1939). 



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