238 THE ANTIOEN-ANTIBODY REACTIONS 



The test has recently been revived by Huddleson and his colleagues (1933) as the 

 opsono-cytophagic reaction and used as a measure of the immune response in brucella 

 infections and pertussis. No attempt is made to measure a precise ratio of control and 

 test opsonic effects, but bloods are arbitrarily graded according to the number of bacteria 

 ingested by a standard number of leucocytes. 



The Toxin- Antitoxin Reaction. 



This reaction hardly falls into the same category as those we have been consider- 

 ing, since the titrations are carried out in vivo. There is, however, no doubt at 

 all that the neutralization of toxin by antitoxin depends on the same factors, and 

 is governed by the same laws, as any other antigen-antibody reaction ; and we 

 have seen that, when toxin and antitoxin are mixed in the test-tube, specific pre- 

 cipitation occurs. It is convenient to discuss in this chapter some of the peculiarities 

 recorded in the in vivo tests, since many of these were noted during the pioneer 

 studies of Ehrlich (1897) on the standardization of diphtheria antitoxin, and so 

 formed the basis of the controversy in regard to antigen-antibody reactions in 

 general. 



The starting-point of any method of standardization is the definition of units 

 of measurement. When these units have to be defined in terms of some in vivo 

 reaction, the resulting measurements are liable to errors of a kind different from 

 those involved in in vitro titrations. These errors and the ways in which they 

 can be avoided or allowed for are discussed in Chapter 43. For the moment 

 we are concerned only with the general nature of the quantitative results that have 

 been recorded. 



The first reagent to which a unit was assigned was the toxin. This unit, the 

 Minimal Lethal Dose, may be defined as follows : 



The Minimal Lethal Dose (M.L.D.) of diphtheria toxin is the least amount that will, 

 on the average, hill a guinea-pig of 250 gm. weight within 96 hours after subcutaneous 

 inoculation. 



Ehrlich (1897) defined the unit of antitoxin in terms of the M.L.D. as the smallest 

 amount of antitoxin that will neutralize 100 M.L.D. of toxin. This left the M.L.D. as 

 the fundamental unit ; but it was soon discarded. Toxin, on storage or on treat- 

 ment with a variety of physical or chemical reagents, has a tendency to lose its 

 toxicity while retaining its combining power for antitoxin. It is converted into 

 toxoid. Under these conditions the definition of a unit of antitoxin, A.U., in terms 

 of the number of M.L.D. of toxin that it will neutralize clearly becomes impossible, 

 since no standard stable toxin can be preserved. An antitoxic serum, when dried 

 iti vacuo and stored at 0° C, remains stable over long periods of time, and hence 

 provides an excellent standard of reference. Ehrlich's original antitoxin has been 

 adopted as an international standard, and the correct definition of a unit of 

 diphtheria antitoxin is as follows : 



One unit of Diphtheria Antitoxin (1 A.JJ .) is contained in that amount of antitoxic 

 serum that has the same total combining capacity, for toxin and toxoid together, as one 

 unit of Ehrlich's original antitoxin. 



The fact that one unit of Ehrlich's original antitoxin happened to neutralize 

 100 M.L.D. of the particular toxic filtrate with which he was working has now 

 only a historical interest. The units of other antitoxins are defined in a similar 

 way, some particular specimen of the antitoxic serum in question being selected 

 as an arbitrary standard, against which all subsequent samples are measured. 



The actual procedure consists in first determining the quantity of a given toxic 



