An Explanation of the Failure of the Direct Antiglobulin Test to Detect Erythrocyte Sensitization in ABO Haemolytic Disease of the Newborn and Observations on Pinocytosis of IgG Anti‐A Antibodies by Infant (Cord) Red Cells

Abstract

S ummary . Group A cord cells sensitized with large amounts of IgG anti‐A/B antibodies will give strong antiglobulin test reactions. The failure of the direct antiglobulin test in cases of ABO haemolytic disease is due to the relatively lower level of antibodies sensitizing the cells. The conjugation of ferritin with IgG anti‐A/B globulin gave an electron dense reagent which was used to study the distribution of A sites on adultand infant group A red cells. Similar contagious patterning of A sites was found on both the adult and infant cells’ surface. The levels of conjugated antibodies on the cells of the different bloods were in agreement with the fact that A 1 adult cells have more A sites than A 2 adult or infant A cells and that A 2 adult cells have similar numbers of A sites to A 1 infant cells. The distribution of A sites on infant and adult cells shows similar degrees of dispersion. Thus the average distance between A sites must be greater on infant A 1 cells than on adult A 1 cells. A combination of the small molecular size of IgG antibodies and the relatively greater distances between the A sites of A 1 infant cells compared to those of A 1 adult cells is proposed to be the cause of the failure of the direct antiglobulin test to give strong reactions with the sensitized cells from ABO HD cases. Pinocytotic activity shown by group A infant cells exposed to IgG anti‐A antibody levels similar to those found in ABO haemolytic disease cases was mainly confined to the more primitive cells containing ribosomes.