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Why is ABO incompatibility between a mother and fetus usually milder than Rh incompatibility?

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Close-up illustration of multiple human red blood cells with their characteristic disc shape, floating against a red background. — blood group The major component of human blood is the red blood cell.

ABO incompatibility between a mother and fetus is generally milder than Rh incompatibility because of the distribution and nature of the antigens involved.

While Rh antigens are primarily restricted to red blood cells, ABO antigens are widely distributed throughout various body tissues, including the lining of blood vessels and the epithelial cells of the gastrointestinal and urinary tracts. Because these antigens are present in many tissues, they act as a "sink" that absorbs a significant portion of the maternal antibodies before they can reach and damage the fetal red blood cells.1

In contrast, Rh incompatibility—specifically involving the D antigen—is more severe because the Rh antigen is found almost exclusively on red blood cells. When a mother produces anti-Rh antibodies, they are not diverted by other tissues and can more effectively target and destroy the fetal red blood cells, leading to more serious cases of erythroblastosis fetalis.23