Short communication structure of 'natural antibodies' in lower vertebrates.

'Natural' antibodies are generally believed to possess solely 2H-2L immunoglobulin structure. They have many disulphide bonds, the structural importance of which is not yet understood. However, some natural heterohaemagglutinins in the sera of some lower vertebrates (eel, shark, perhaps also trout and toad) are subunit proteins; the subunits are held together not by disulphide bonds but solely by non-covalent interactions like animal lectins. The relevance of these findings for immunophylogeny is discussed.