detection of abnormal molecular patterns, cell stress, aberrant cellular prolif-
eration, and/or damage caused to the organism (Muraille 2013).
These last two activities (filtering over presence and promotion of coopera-
tion) contribute to the cohesion of the biological individual (Figure 3.1). They
strengthen functional integration (interconnection and interdependence), inter-
nal interactions (and therefore near-complete decomposability), as well as
cooperation and absence of conflict between components.
Two important additional remarks are in order. First, the immune system is
truly systemic, which means that, contrary to many bodily systems (e.g.,
digestive or respiratory), the immune system exerts its influence throughout
the body. Therefore,filtering over entry,filtering over presence, and promotion
of cooperation occur continuously and everywhere in the organism. (Many
organs, either located at the interface with the environment or more internal,
have been called“immunoprivileged”–for instance, the central nervous system
and the eye, among many others. Yet recent research has shown that such
compartments are not without immune influences (Mellor and Munn 2008;
Louveau et al. 2015a). They are instead places where immunological processes
are regulated differently than in the rest of the organism.)
Second, one crucial characteristic of the immunological approach to biolo-
gical individuality is its broad range of application. It applies, in fact, to the
whole living world, since immune systems are found in virtually all living
things, including prokaryotes, plants, invertebrates, and vertebrates (see
Section 2).
Thus, the immune system plays a central role in the delineation of the
boundaries of a living thing and the determination of the cohesion between its
constituents (Pradeu 2012). But is this account really different from the self–
nonself framework examined above? It is actually essential to understand how
the two accounts differ. For Burnet, the self–nonself framework is scientifically
testable and useful if and only if self and nonself are defined in terms oforigins:
the self is what originates from the organism once it has acquired the capacity to
recognize its own constituents, while the nonself is everything that does not
originate from the organism. In stark contrast, the discrimination proposed
above between what is part of the living thing and what isn’t is not based on
a question of origin, since in this view many genetically foreign entities can be
accepted by the immune system while many endogenous entities are routinely
destroyed. For example, in the account developed here, microbiota components
that are immunologically tolerated belong to the organism, while they are
considered nonself and therefore not part of the organism in the self–nonself
theory. In other words, I propose that the immune system contributes to the
continuously re-delineated distinction between the“inside”and the“outside”
Philosophy of Immunology 23