Functional ascription in biology is contentious, particularly when it takes a teleological form, that is, when the question being raised is the “purpose” of a biological item (a trait, an organ, or a system). Is it legitimate to ask what a trait, an organ, or a system is “supposed to do”? Traditional physiological “systems” (digestive, respiratory, and so on) are a typical case of functional ascription: it is often said, for example, that the function of the digestive system is to ensure digestion. More specifically, among biological systems, the immune system has been used in the literature on biological functions as a paradigmatic example of functional ascription in a teleological sense (Matthen M, Levi E, J Philos 81(7):351–372, 1984; Melander P, Philos Sci 60(2):223–241, 1993). One important corollary of that analysis is that autoimmune diseases have been seen as an example of a dysfunction: when an autoimmune disease occurs, the argument goes, we can say that the immune system does not do what it is “supposed to do.” The aim of this chapter is to put into question the usefulness of this etiological approach to the immune system and to defend instead a systemic approach to the immune system< Leer menos