Quantitative Measurments Concerning A and B Antigen Sites

Summary Using 125 I‐labelled anti‐A and anti‐B, the number of A and B antigen sites per red cell for samples obtained from adults of different phenotypes was estimated to be: on A 1 cells, 0.81—1.17×10 6 ; on A 1 B cells, 0.46—0.85×10 5 ; on A 2 cells, 0.24—0.29×10 6 ; on A 2 B cells, 0.12×10 6 . The number of sites on A 1 cells obtained from cord blood was 0.25—0.37×10 6 , and on the single example of A 2 cord cells were 0.14×10 8 sites. The number of B sites on adult cells were as follows: on B cells, 0.61—0.83×10 6 ; on A 1 B cells, 0.31—0.56×10 6 sites. The equilibrium constants and the rates of dissociation of the reaction between anti‐A and A 1 and A 2 cells were also determined. The value of the equilibrium constant was approximately three times as great and the rate of dissociation three times as slow with A 1 cells compared to A 2 cells. This is consistant with the view that theere is a small difference in the molecular structure between the A 1 and A 2 antigen.