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Transferrin (Tf) is a vertebrate plasma protein that binds two Fe super(3+) ions in a bicarbonate dependent fashion. Tf functions in the transport and delivery of iron to cells via transferrin receptor-mediated endocytosis. As iron is required for the growth of many bacteria and free Fe super(3+)/Fe super(2+) is involved in the generation of chemically damaging hydroxyl free radicals, Tf also functions to limit bacterial infections and protect against iron toxicity. Lactoferrin (Lf) is an Fe super(3+)-binding homolog of Tf present in milk and secretions that bathe mucosal surfaces. Lf is also secreted by neutrophils and is believed to play a role in modulation of immune and inflammatory responses.