Any substance produced by an organism in order to prevent freezing of its tissues or body fluids when subject to subzero environmental temperatures. Many animals living in cold climates adopt a strategy of preventing ice formation in their tissues when subject to freezing conditions. One way of achieving this is to accumulate solutes in their blood, thereby raising the osmotic concentration and so depressing the supercooling point. Salts and sugars contribute to this, but organisms also produce relatively inert molecules, notably glycerol and other polyhydric alcohols (polyols), such as sorbitol and ribitol, specifically for this purpose. For example, high concentrations of glycerol can enable the survival of certain cold-hardy invertebrates at temperatures as low as −60°C. Certain vertebrates, fungi, plants, and bacteria manufacture antifreeze peptides or antifreeze glycopeptides, which are effective antifreeze agents at relatively low concentrations. They bind to the edges of ice crystal lattices and prevent the addition of further water molecules, causing a phenomenon termed ‘thermal hysteresis’, in which the freezing point is depressed well below the melting point—hence these peptides are also called thermal hysteresis proteins. Frost-tolerant plants increase the concentration of sugars in their cells in preparation for winter. The sugars help to prevent water loss from the cells when the water content in the cell walls is reduced by freezing. See also cryoprotectant.