ANTIGENS AS BIOCATALYSTS 33 



a mean titre during the first 50 days from 880-1630 for the three 

 rabbits; and with a single dose of 10^ bacilli per k.b.w. three rabbits 

 showed a highest agglutinating titre of 130 to 510 and a mean titre of 

 37 to 280 during the first 50 days for a second series of rabbits. Thus 

 Topley finds that a dose of 10^ bacilli per k.b.w. is in the neighborhood 

 of the threshold dose for a detectable response. 



To determine the actual amount of antigen contained in a dose of 

 this order a dense bacterial suspension was dried and brought to con- 

 stant weight over calcium chloride; and the dry weight of 10*^ bacilli 

 was found to weigh 0.55 mg. A dose of 10^ bacilli thus corresponded 

 to about 0.000,005 mg. of solid material. When allowance had been 

 made for the possible presence of non-bacterial material brought in 

 with bacteria by washing out the agar in preparing the suspensions, 

 and for the flagellar material which could not have contained very 

 much antigen, it would seem probable that the dose of active sub- 

 stance administered with 10^ bacilli was of the order of 0.000,000,5- 

 0.000,005 mg. After such a consideration Topley used a conservative 

 figure of 50 ml. of serum contained in the blood of a rabbit of 2000 g. 

 and evaluated its agglutinating titre as 1 : 500 or more. He thus pre- 

 sented a quantitative study to emphasize the well known and striking 

 disproportionality between the amount of antigen injected and the 

 amount of antibody produced. 



Fiooker and Boyd (1931) using the data obtained by Topley carried 

 out calculations to determine the amount of antibody produced by a 

 single antigen molecule. They assumed that the above mentioned 

 weight of material— as evaluated by Topley— present in 10^ bacteria 

 as the minimal effective dose of active antigenic substance is of the 

 order of 5X10~^° gram per k.b.w. of rabbit having an agglutinating 

 titre of serum 1 : 500. Considering that the fundamental specific anti- 

 genic unit has a molecular weight of 20,000— a very conservative 

 figure— they arrived at the following conclusions: 



One gram of material will contain, after Avogadro, 5X10"^ 

 (6.06X10-^)=3X10^^ molecules, and the minimal eff^ective dose of 

 antigen (5X10~^*^ g.) per k.b.w., contains therefore approximately 

 1.5X10^" molecules. 



One ml. of serum from rabbit 11 responsive to 8.3X10^° molecules 

 in Topley's experiments contained 5000 or more agglutinin units. 

 A "unit" was estimated to affect the agglutination of 2X10^ bacteria 



