The ancestor of the adaptive immune system was the CAM system for organogenesis.
The adaptive immune system as we understand it is uniquely possessed by vertebrate species and by none other. Nevertheless, it is generally thought that this system represents the ultimate sophistication of much older cell-cell interaction mechanisms. In this paper, I have suggested that plasma membrane cell adhesion proteins that were involved in ontogenic organogenesis since time immemorial were the ultimate ancestor of the adaptive immune system. N-CAM of the chicken for neuronal organogenesis already possesses four beta 2-microglobulin-like domains and it is this domain from which the adaptive immune system sprang. The germ for this beta 2-microglobulin-like domain is already found in the cell adhesion protein of the slime mold straddling the border between unicellular and multicellular eukaryotes. A slightly different version of the above view has recently been expressed by my colleagues.
Ohno
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Beckman Research Institute of The City of Hope, Duarte, Calif.
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D005075:Biological Evolution; D015815:Cell Adhesion Molecules / Q000235:genetics* / Q000276:immunology; D002645:Chickens / Q000235:genetics; D004023:Dictyostelium / Q000235:genetics / Q000276:immunology; D006801:Humans; D007107:Immune System / Q000502:physiology*; D008969:Molecular Sequence Data; D011487:Protein Conformation; D012689:Sequence Homology, Nucleic Acid; D001613:beta 2-Microglobulin / Q000235:genetics* / Q000276:immunology
D015815:Cell Adhesion Molecules; D001613:beta 2-Microglobulin