become extremely challenging nowadays. Focusing on defense against patho-
gens would be too narrow. On the other hand, extending the definition of
immunity so far as the overall physiological regulation of the body (as some-
times suggested by recent studies on the role of the immune system in metabo-
lism, tissue repair, homeostasis, development, and so on) would be so broad that
it might cease to be scientifically fruitful, as almost everything in biology could
be said to be immunological, at least to some degree. One possibility would be
to say that any component involved in a series of activities thatincludesdefense
against pathogens is part of immunity (in this conception, immunity would be
centered on, but not limited to defense), but this option would still leave us with
a broad conception of immunity. Uncertainties of this kind have led to
a situation in which, perhaps unsurprisingly, current immunologists do not
offer a single and consensual response to the question“What is immunity?”
I have tried in the past to put forward a definition of immunity according to
which immunological processes correspond to all the processes of biochem-
ical recognition of a target, followed by the elimination or the regulation of
the elimination of this target (Pradeu 2012). This definition is not without
difficulties (particularly concerning the level(s) at which it is supposed to
apply), but I think it does capture the diversity of phenomena currently
studied by immunologists, while remaining sufficiently specific so as not to
include all classically understood physiological processes occurring in the
organism. It also has the advantage of reflecting the diversity of immune
systems in nature, from prokaryotes to vertebrates. More research is needed,
though, to determine whether a definition of this kind could satisfy the
majority of immunologists.
A related but broader and more imprecise characterization of immunity is to
say that immunity is one of the main devices insuring the cohesion of the
organism and the delineation of its boundaries. We will explore and assess
this view in the next section, which is devoted to immunology’s contribution to
the definition of biological individuality.
In summary, this section has shown that immune defense is essential in every
living thing, but immunity cannot be reduced to defense.
3 The Unity of the Individual: Self–Nonself, Autoimmunity,
Tolerance, and Symbiosis
In 1998, Clint Hallam, a patient from New Zealand who was operated on in
Lyon, France, received the world’sfirst hand graft (Dubernard et al. 1999). The
operation was a technical success, and initially the recipient seemed to feel all
right. Yet he soon started to consider as“other”(foreign) the transplanted hand,
Philosophy of Immunology 13