NATURE OF ANTIBODIES 201 



quantity of antitetanus soruiii necessary to save tlie life of a 350 

 gm. guinea pig for ninety-six hours against the official dose of 

 standard toxin furnished by the Hygienic Laboratory of the 

 Public Health and Marine Hospital Service." Compared to a 

 unit of diphtheria antitoxin it has slightly more than ten times 

 the protective power. The method of standardizing tetanus anti- 

 toxin was suggested by Rosenau and Anderson (1907). As men- 

 tioned above, the Ramon technique is now used extensively. 



Scarlet fever antitoxin is measured in terms of its protective 

 value for skin test doses of toxin in accordance with the work of 

 Dick and Dick (1924). 



Agglutinins. — In determining the titer of a clumping or ag- 

 glutinating serum, as, e.g., one that agglutinates E. tiiphom, 

 the antigen is not diluted, as in the precipitin ring test. On the 

 contrary, a series of serial dilutions of the immune serum is made. 

 A number of small test tubes are placed in a rack and to each is 

 added a uniform and measured amount of the various dilutions 

 and an equal amount of a standard turbid suspension of the 

 antigen, e.g., E. typhosa. After shaking and incubating, nota- 

 tion is made of the highest final dilution of the immune serum con- 

 taining the antibody that gives perceptible clumping, and this 

 is the titer of the immune serum. Titers as high as 1 :5,000 or 

 1 :10,000 are readily produced in rabbits by vaccination. The 

 suspension of bacteria used in agglutination work is usually diluted 

 to match a known turbidity standard. 



Precipitins. — The strength or titer of a precipitating immune 

 serum is commonly described in terms of the highest dilution of 

 the antigen that gives a perceptible ring precipitate when strati- 

 fied over the undiluted immune serum in a small test tube. Titers 

 as high as 1 : 1,000,000 have been produced. 



Cannon and Marshall (1940) have called attention to an old 

 but more rational metJiod of determining the titer of precipitating 

 antibodies. They prepare a standard suspension of collodion 

 particles, film them with antigen and mix an equal volume of the 

 filmed particles with a corresponding volume of serial dilutions 

 of the antibody as in an agglutination test. After incubation for 

 30 minutes at 37° C. the tubes are centrifuged at low speed to 

 bring together antigen-antibody coated particles. The contents 

 of the tubes are then resuspended and examined for agglutination. 



