What factor allows some blood group antibodies to hemolyze appropriate red cells?

The factor that allows some blood group antibodies to hemolyze appropriate red cells is complement. When antibodies bind to specific antigens on the surface of red blood cells, they can initiate the complement cascade—a series of proteins in the blood that enhances the immune system's ability to clear pathogens or damaged cells. This activation of complement can lead to the formation of membrane attack complexes that create pores in the red blood cell membrane, ultimately causing lysis or destruction of the targeted cells.

In contrast, anticoagulants are used to prevent blood from clotting, preservatives are added to blood products to extend their shelf life, and penicillin is an antibiotic that targets bacteria rather than playing a role in the hemolysis of red blood cells. Therefore, complement is specifically associated with the process of hemolysis in the context of blood group antibodies.