Quantitative and thermodynamic measurements on I and i antigens of human red blood cells.

Different homogeneous IgM cold agglutinins (two anti-I, two anti-i and one anti-Il cross-reacting antibodies) have been used to determine the antigen site densities of adult I and i erythrocytes and of cord red cells. The equilibirium constants and the thermodynamic constants of these reactions have been determined. The two anti-I antibodies, which did not combine with i or cord red blood cells, recognized two different determinants on I red cells. The antigen density of the I Fla. receptor was 120,000 sites per erythrocyte and the standard enthalpy change (-deltaH degrees) of the reaction was 18 to 25 kcal/mole. The antigen density of the I Loi. determinant varied according to the red cells tested and the enthalpy change (-deltaH degrees) of these reactions was 50-65 kcal/mole. The i and cord erythrocytes reactive structures were more heterogeneous than those present on I erythrocytes. The equilibrium constants rapidly decreased as the temperature rose and the standard enthalpy changes (-deltaH degrees) ranged from 50 to 90 kcal/mole. Two types of i determinants were observed; one of the anti-i antibodies reacted mainly with an i component present on cord erythrocytes, the other antibody reacted with a different i component present on i adult red-cells. The determinants, recognized by the cross -reacting antibody on I red cells, differed fromthose on i or cord red cells in equilibrium constant, thermodynamic constants, index of heterogeneity of the reaction and in their sensitivity to formalin treatment.