IAT is used to detect and identify antibodies. The test uses antihuman globulin (AHG) to detect in vitro sensitization of red cells. Patient serum or plasma is incubated with reagent red cells with known antigen phenotypes. If an antibody is present in the serum, it will bind to the reagent red cells with the corresponding antigen. Because IgG molecules are incapable of producing macroscopic agglutination, AHG is needed to act as a bridge. The AHG used in the IAT is Anti-IgG. Anti-IgG will bind to the patient's IgG antibody if present and facilitate macroscopic agglutination. IgG antibodies present in the patient's serum or plasma are considered clinically significant.