Levi has a nice post in response to my post on Luhmann. It does a good job of explaining a bit more of the back ground on issues like risk, systems/environment, immunity, etc. However, it has also made me think that a longer post on immunity and autoimmunity might be needed over here.
For me, as well as for many other thinkers, the issues of biopolitics are bound up with the issues of immunity. For example, in Foucault we are given schematics of power that correlate to responses of disease. So, Foucault explains that responses to leprosy elucidate sovereign power, responses to plague elucidate disciplinary power, and responses to smallpox elucidate governmentality and biopower. But something else emerges in Derrida and in Esposito, rather than responses to certain diseases being models or instances of different logics of power, immunity becomes a way of thinking the social itself, a metaphor or map for community and violence. Esposito warns us that in Derrida's formulation of immunity, there seems to be no space (or only an infinitesimal space) between immunity and autoimmunity. I think this is why Esposito turns more and more to Luhmann, for whom immunity provides both a positive and negative influence on a system.
However, as Miller points out (after prompting from Mitchell) in For Derrida, autoimmunity is a "figure of a figure" (p. 124), or a metalepsis. What does that mean? Well, it means that the concept of immunity isn't particularly a natural one, but already a metaphor used to understand the way particular systems of our bodies work. In this case, I really suggest Donna Haraway's "The Biopolitics of Postmodern Bodies" and Ed Cohen's A Body Worth Defending. In Haraway's case you get an interesting story about the way certain discourses of the immune system work within the coordinates of a military-industrial complex. In Cohen you receive a longer work about how the very idea of immunity and self-defense (two different historical concepts, as Cohen points out) arise and go on to explain the notion of the way the body protects itself. This all means that certain societal arrangements became a figure for understanding the body, and that certain ways the body regulates itself has become a figure for understanding society.
This metalepsis of immunity is something we have to grapple with if we want to seriously understand the concept of immunity. This becomes all the more true if so much is suppose to depend on this issue of immunity; questions of killable and protected, of community and estrangement, of self and other.
 See Discipline and Punish and Abnormal for the first two examples. The smallpox example is in Security, Territory, Population.
 The major texts that Derrida deals with immunity are "Faith and Knowledge", in Philosophy in a Time of Terror, and in Rogues [Thanks to Matt for reminding me]. Hagglund's Radical Atheism is rightly mentioned as a great source on this concept, but I also suggest J. Hillis Miller's For Derrida and W.J. Thomas Mitchell's "Picturing Terror".
 Esposito's notions on immunity is worked out in his trilogy Immunitas, Communitas, and Bios. Also in his most recent book, Third Person.
 Haraway's work provides a nice contrast to Derrida, as well. Derrida's concern is the way that immunity slips into auto-immunity, but Haraway's essay is provoked by the death of a close friend who died of AIDS (immuno-suppression, not autoimmunity). Also, she has an interesting footnote where she observes that obsession with autoimmunity seems to come at the cost of concerns over things like parasites, which leads us to not "take responsibility for the differences and inequalities of sickness globally." (p. 252, n. 2)