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Antibody type responsible for ABO blood group reactions is IgM. ABO antigens are carbohydrate structures on red blood cells, and people naturally develop anti-A and anti-B antibodies without prior exposure. IgM fits this role well because it is produced early in an immune response and exists as a pentamer, giving it high avidity and the ability to bind multiple antigens at once. This structure makes IgM especially effective at causing agglutination of donor red blood cells and activating the complement system, which leads to rapid hemolysis when incompatible blood is transfused. In general immune responses, IgM is the first antibody produced before switching to IgG, so it dominates the initial reaction to new ABO antigens. While IgG can contribute in some cases and the other immunoglobulins have different roles (IgG in later responses and placental transfer, IgA in secretions, IgD as a B cell receptor), IgM is the primary driver of ABO reactions.