Final answer:
IgG antibodies are allowed to bind to red cells by incubation at 37°C (body temperature). This temperature allows for IgG-mediated immune responses, although it may not cause visible agglutination of red cells, which can be detected with Coombs' reagent.
Step-by-step explanation:
IgG antibodies are allowed to bind to red cells by incubation at 37°C (body temperature). At this temperature, the IgG antibodies bind to cellular antigen, which can lead to complement activation and cell lysis. IgG antibodies can also mediate Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC) with cytotoxic T cells, natural killer cells, macrophages, and neutrophils. While incubation at this temperature enables the binding, IgG bound to red blood cells may not cause visible agglutination due to the size of the cells in relation to the antibody's binding sites. The presence of these antibodies can, however, be detected through the use of Coombs' reagent, which links the IgG attached to neighboring red blood cells to promote agglutination.