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Antibodies, often monoclonal, in which the two antigen-binding sites are specific for separate antigenic determinants. They are artificial antibodies produced by chemical crosslinking, fusion of hybridoma cells, or by molecular genetic techniques. They function as the main mediators of targeted cellular cytotoxicity and have been shown to be efficient in the targeting of drugs, toxins, radiolabeled haptens, and effector cells to diseased tissue, primarily tumors.