A poison produced by a living organism, especially a bacterium. Endotoxins are lipopolysaccharides derived from the outer membrane of Gram-negative bacteria, such as strains of Escherichia and Salmonella. They are released only when the bacterial cell dies or disintegrates and are seldom fatal. Exotoxins are secreted by a bacterial cell into the surrounding medium and are generally highly toxic to the host. For example, botulinum toxin produced by Clostridium botulinum is one of the most poisonous substances known. Eukaryotes can also produce toxins. Toxic ‘algal blooms’ of dinoflagellates release various neurotoxins into seawater, poisoning fish and shellfish and also humans who consume affected seafood (see Dinomastigota). Ergot poisoning is the result of eating grain contaminated with toxins produced by the fungus Claviceps purpurea. In the body a toxin acts as an antigen, producing an immune response. High-affinity IgG and IgA antibodies bind to the receptor-binding sites of secreted bacterial exotoxins, thereby neutralizing them by preventing them from binding to receptors on host cells. Such antibodies are called neutralizing antibodies.