Which reaction is caused by an IgG antibody binding to the surface of a red blood cell?

The reaction caused by an IgG antibody binding to the surface of a red blood cell is agglutination. This occurs when antibodies bind to antigens on the surface of red blood cells, clumping them together into visible aggregates or clusters. This process is significant in various clinical settings, particularly in blood transfusion compatibility testing and certain autoimmune hemolytic anemias.

When IgG binds to red blood cells, it forms bridges between cells, supporting the concept of agglutination, where the binding of antibodies leads to the visible clumping of cells. This is a crucial part of the immune response, allowing the immune system to more effectively identify and clear foreign cells or pathogens.

The other types of reactions listed involve different mechanisms. Precipitation involves soluble antigens and antibodies forming visible complexes, while sensitization refers to the initial binding of antibodies to their antigens, without the formation of visible aggregates. Neutralization pertains to antibodies binding to pathogens or toxins, inhibiting their biological effects, but does not directly involve cell clumping. Thus, agglutination is the most appropriate term for this scenario involving IgG antibodies and red blood cells.