In immunology, which of the following antibodies is most associated with a transfusion reaction?

The antibody most associated with a transfusion reaction is IgM. This is due to its structure and function within the immune system. IgM is a pentamer, meaning it consists of five antibody units joined together, which gives it a much higher avidity for antigens compared to other antibodies.

When blood is transfused, if the donor blood type differs from the recipient's, the IgM antibodies in the recipient's immune system can recognize the foreign red blood cells and bind to the ABO antigens present on their surface. This binding triggers the activation of the complement system, leading to cell lysis and hemolysis of the transfused red blood cells, resulting in an acute transfusion reaction.

IgG, while involved in many immune responses and capable of crossing the placenta, typically mediates delayed-type transfusion reactions rather than acute ones. IgA plays a prominent role in mucosal immunity but is not significantly involved in transfusion reactions. IgE is primarily associated with allergic responses and protection against parasitic infections, not with the mechanisms of transfusion reactions.

Thus, IgM's role and mechanism make it the antibody most closely associated with acute transfusion reactions.