transition from unicellular to multicellular organisms where, again, the constitution
of an immune system at the multicellular level was crucial (Michod 1999;Pradeu
2013). Obviously, this approach to the evolution of immune systems is entirely
different from the classic (and naturally important) discussion in immunology over
how adaptive immune systems evolved in vertebrates some 400 to 500 million
years ago (Pancer and Cooper 2006). Much work remains to be done on this
question of the origins of immunity, including by developing cross-species com-
parative immunology.
What has been said here about the expansion of the scope of immunity can also
be framed in terms of the long-standing debate in the philosophy of biology about
functions (Wright 1973;Cummins 1975). Bodily systems generally have been
viewed as obvious instances of a valid functional ascription: the function of the
digestive system is to digest, the function of the respiratory system is to breathe,
and so on. According to the traditional view of immunity, the function of the
immune system is to defend the organism. Following this tradition, some philoso-
phers have suggested (Matthen and Levy 1984; Melander 1993) that the function of
the immune system, in the etiological sense (that is, what it has been selected for)
(Wright 1973), is organism defense. Yet saying what the immune system has been
selected for proves, in fact, extremely difficult. Given the various activities of the
immune system and the diversity of selective pressures it has undergone, it would
be inadequate, or at least too restricted, to say that the immune system has been
selected for its capacity to defend the organism against pathogens. It is not possible
to single out one of these aspects and say that this is“the function”(or even“the
main function”) of the immune system. It seems likely that the systemic approach
to functions (Cummins 1975) is certainly better suited for immunology, but
a detailed analysis of the promises and limitations of a systemic approach to
immune functions remains to be done.
2.5 Is It Still Possible to Offer a Precise and Simple
Definition of Immunity?
The extension of the scope of immunity well beyond defense affects dramati-
cally the meaning of the term immunity and raises a thorny but central problem:
given the diversity of phenomena in which the immune system plays a crucial
role, is there still a unity of the concept of immunity, or is it just the loose and
artificial putting together of a horde of processes under a single term? In other
words, is there ultimately such a thing as immunity that unifies all of these
processes under a single concept? I see this question as one of the most
fascinating that current and future immunology must face. Defining what counts
as an immune process and delineating thefield of immunology has indeed
12 Elements in the Philosophy of Biology