Adaptive immunity across the animal kingdom

Yesterday I went to a talk by Jim Kaufman from Cambridge which I found very informative. Textbooks on immunology usually concentrate largely on the human immune system and the immune system of the mouse, as the main experimental animal. For a long time it was believed that adaptive immunity was confined to vertebrates (with a few exceptions like lampreys which appeared to lack it). Those animals which have adaptive immunity seem to have the whole package, in the sense that they have antibodies, T cells and MHC molecules. So it looks like that from an evolutionary point of view all these things arose about the same time (on the time scales typical for evolution). In this talk I discovered that very many invertebrates have adaptive immunity. They use different systems from that familiar from vertebrates and often different systems from each other. Insects are included. Drosophila, for instance, has an adaptive immune system based on a single gene which can be spliced in a huge number of different ways. From the references given in the talk it looked to me as if these facts about invertebrates came out in about the last five years.

One of the main themes of the talk was that there is a big divide in adaptive immunity between mammals and other vertebrates. (There is a marsupial which is an exception.) The difference concerns the major histocompatibility complex. The MHC molecules in chickens (to take the principal example discussed in the talk) are more adapted to specific pathogens than is the case in mammals. This might also mean that mammals are particularly susceptible to autoimmune diseases. Apparently not so much is known about autoimmune dieases in non-human mammals or non-mammalian vertebrates. This is understandible from the point of view of the motivation of human beings. Kaufman also pointed out that not very much is known about the frequency of autoimmune diseases in poorer countries.

Another theme of the talk was the evolution of adaptive immunity. I was not able to follow the details very closely. One idea was that T cells appeared relatively early and may have recognized targets directly before there were cells to present them with antigens. Another was that natural killer cells were one of the earliest elements which led to adaptive immunity. In any case I enjoyed the opportunity to obtain an unusually broad view of immunity.

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