An anti-I cold auto-agglutinin enhanced in the presence of sodium azide.

Difficulty in determining the ABO blood group led to the discovery of an anti-I cold auto-agglutinin in the serum of a blood donor. The peculiarity of this antibody is that its activity is enhanced in the presence of a preservative found in manufactured anti-sera: sodium azide. At 4 degrees C, the auto-antibody agglutinates RBCs even without NaN3 but the drug increases its effect in the cold. At 22 degrees C, the drug is necessary for the adsorption of the antibody on red cells.