Evidence relating to the problem of the origin of natural antibodies, especially hemagglutinins and hemolysins has been critically reviewed. Based on this analysis it is concluded that natural antibodies are, with possibly rare exceptions, of immune origin. Natural antibodies, like those generally conceded to be of immune origin such as Rh antibodies, occur in the univalent as well as the bivalent form. Natural immunity to bacteria and viruses is generally conceded to be due to undiagnosed or symptomless infections, since such immunity is most frequent among individuals who have been exposed to the disease or who carry the micro?rganism in their body. The peristence of natural antibodies throughout life is explained by experiments on Rh sensitization showing that antibodies induced by deliberate immunization likewise may persist indefinitely after all contact with the antigen has been discontinued.Natural antibodies for red cells, such as the blood group antibodies and cold hemagglutinins, are shown to be of heterogenetic immune origin and are attributable to the presence of related antigens in bacteria and animal parasites. Polysaccharides of similar chemical structure have been isolated from capsules of penumococci as well as from secretions possessing blood group specificity, thus providing a chemical explanation for such heterogenetic reactions.
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