Talmudic Question #18

A good TQ is one that invites another question. We previously asked: Invertebrates are said to have only innate immunity. Why do you suppose they have not evolved acquired immunity, as vertebrates have done?  Let us now turn it around and ask: Why have vertebrates evolved acquired immunity?

Comments

[Amber]

I think this TQ is a very interesting question (and a significant one for the squid-vibrio system). I'd answer this question with ANOTHER question: What is different about the microbial interactions vertebrates experience compared to invertebrates?

[Ian]

I missed the previous question, and I disagree with the premise that inverts don't have an acquired immune system. There's good evidence that inverts do have something conceptually similar to an adaptive immune response; for example, the large number of splice variants of Dscam in insects (Science. 2005;309(5742):1874-8.). Lampreys, also, were considered to not have an adaptive immune response and don't in fact have a standard vertebrate immune response, but do have a quite different but parallel system of combinatorial complexity that looks very much like an adaptive response (Science. 2005; 310(5756):1970-3.). The recent paper on the sea urchin immune system (Science 2006;314(5801):952 - 956) says:

Recent findings suggest that somatic mechanisms of receptor diversification analogous to those of the acquired system of jawed vertebrates may be a more widespread feature of animal immunity than previously supposed. Examples of these include a gene conversion–like process that diversifies variable leucine-rich repeat (LRR)–containing receptor (VLR) proteins in jawless vertebrates (9, 10), somatic mutation of fibrinogen-related protein (FREP) receptors in a mollusc (11), and extensive alternative splicing of the Down syndrome cell adhesion molecule (DSCAM), a molecule that principally guides neuronal patterning, to generate immune reactive isoforms in insects (12, 13). On the basis of this narrow sampling, it is likely that a universe of novel and dynamic immune mechanisms exists among the invertebrates, further validating their role as significant immune models.

Given that, the answer to your question "Why have vertebrates evolved acquired immunity?" is that for vertebrates, as for invertebrates, an adaptive immune response is a useful way to respond to repeated infection -- so useful, in fact, that it's been rediscovered many times, in many different forms.

That makes today's question a less interesting one, I think. The question becomes an extension of the previous one, and just focuses on the "as vertebrates have done part" -- why have vertebrates gained the particular form of adaptive immunity that we use? Here the answer is very likely, "Chance". Adaptive immunity arose pretty much full-fledged in sharks. It's (probably) in this form because the recombinase genes that sharks and their descendants use (RAGs) are descended from a particular transposon, and it only happened to jump into the right place and take on the right function in sharks (Proc Natl Acad Sci U S A. 2006 Mar 7;103(10):3728-33) -- a fortuitous accident that happened to work pretty well, but perhaps not as much better than the alternatives as we've been thinking.