"The Acquired Immune System: A Vantage from Beneath", Hedrick 2004 (immune systems as parasites; excerpts)

I absolutely loved this. The concept of the adaptive immune system as something that gives the ability to get a slight advantage over your conspecifics, at the expense of selecting your pathogens to be more virulent such that loss of the adaptive system becomes fatal, reminds me forcefully of all sorts of things in prokaryotic and eukaryotic genome structure. Things that happen because they can and then get locked in place by other things built on top of them even though they themselves are harmful, or sheer selfish elements. Like poison/antidote pairs of genes in bacteria that stick around even though they increase average generation time because fluctuations in the levels of the two make a small fraction of bacteria grow extra slowly and be stress-resistant, or the evolution of the spliceosome to make sure self-splicing introns always leave leading to vertebrate genes that are 90% spliceosome-requiring introns, or the sheer abundance of transposons that make up more than half of our genomes...