Though our society (that is American society) makes a great rhetorical show of cherishing "diversity," I've said several times that it is on the HLA locii that this takes real form (see here or here). The polymorphic diversity seems peculiar on such essential genes implicated in the coding of crucial components of the immune system. They shoud have been "fixed" long ago in the variants which code for the most "fit" phenotypes. But, as this article suggests, the diversity of the HLA locii is another twist in the great game between parasites and their prey (sex is likely another arrow in the quiver of multicelluar organisms against their foes). In Narrow Roads of Gene Land William Hamilton even suggests that balancing selection, that is, advantage to novel heterozygote conformations, might not be where the advantage lay, rather the long term fitness is paramount in that parasites will sweep through populations cyclically and any one defense is never absolutely perfect. A mixed strategy, if at any one time suboptimal, might be the most prudent. But in any case, the crucial point about the aforementioned BBC article is that it points out that a particular HLA variant on a particular gene might be more efficient at zeroing in on the HIV virus. Remember, this kaleidoscopic formation of genes transcends the bounds of species and predates our separation from our great ape kin, it's got to be doing something right....

Of course, to some extent this is old news.

Addendum: Nature, which is publishing the research mentioned in the article above, has a gene map of the MHC (PDF) molecule. For immune system nerds only....