Polyspecific antihuman globulin (AHG) reagents are used for routine alloantibody detection, compatibility tests and the direct antiglobulin test (DAT). PolyspecificAHG contains antibody to human IgG and the C3d component of human complement, and may also react weakly with IgA and IgM molecules due to anti-light chain activity. A positive DAT generally indicates that the red cells are coated in vivo with immunoglobulin and/or complement. The most important function of the AHG reagent is to detect the presence of IgG-antibodies. The importance of anti-complement in the AHG reagent is debatable since antibodies detectable only by their ability to bind complement are rather rare. However, anti-C3d activity is important for the direct antiglobulin test in the investigation of autoimmune hemolytic anemia (AIHA).