People with Type O blood have a natural defense against severe malaria.

Malaria is caused by Plasmodium falciparum, a parasite that infects red blood cells and forces them to express a protein called PfEMP1. This protein acts like glue, causing infected cells to stick to uninfected cells in a process known as rosetting. Rosetting leads to microvascular obstructions, which can cause organ failure or cerebral malaria.Research led by Mats Wahlgren at the Karolinska Institutet in Sweden has shown that the A and B antigens act as core receptors for this sticking process. In Type A or B blood, the parasite easily bonds with these antigens to form large, dangerous clumps. Because Type O blood lacks these specific A and B antigens, the bonds are much weaker and the clumps are significantly smaller.This biological difference provides a survival advantage in malaria-endemic regions like sub-Saharan Africa. A 2007 study published in the Proceedings of the National Academy of Sciences (PNAS) found that Type O individuals are up to 66% less likely to develop severe, life-threatening malaria. Over thousands of years, this 'balancing selection' has ensured that Type O remains the most prevalent blood group in areas where malaria is a constant threat.The discovery of this mechanism helps explain the geographic distribution of ABO blood groups globally. It also provides researchers with insights into how to develop new treatments that mimic the protective effects of Type O blood. By understanding how the parasite interacts with blood sugars, scientists hope to create drugs that prevent the deadly clumping of red blood cells.