Complement is not activated in ABO-haemolytic disease of the newborn.

We studied the lysis in vitro of group A red cells by IgG anti-A. IgG anti-A, which strongly lysed A red cells from adults, did not lyse A red cells from cord blood, if fresh cord serum from a child with blood group AB was used as a source of complement. In cases of haemolytic disease of the newborn due to A-O or B-O antagonism with a positive direct antiglobulin test with anti-IgG serum, the red cells did not react with anti-complement sera. Apparently, complement is also not activated in vivo in case of A-O haemolytic disease of the newborn.