but this distinction is not to be confused with the opposition between the
“endogenous” (what comes from the inside) and the “exogenous” (what
comes from the outside). The resulting living thing does not coincide with the
traditional self and cannot be accounted for by Burnet’s self–nonself theory.
A related idea is that the process of dualfiltering presented here is dynamic
and never-ending. There is, therefore, a constant re-delineation, through the
action of the immune system, of the constituents and boundaries of a living
thing. This sheds a crucial light on thediachronicidentity of biological indivi-
duals (Wiggins 2016). An entity that is part of a living thing at a given moment
(for example, an immunologically tolerated virus) can cease to be part of that
living thing later in time (for example, if this virus is eventually eliminated by
the immune system). The resulting individuality is relative and changes con-
tinuously, but the criterion used to establish what is part and what is not part of
a living thing remains constant and offers a precise delineation.
3.4 The Role of the Immune System in Turning a Set of
Heterogeneous Constituents into an Integrated Individual
Immunology helps us better understand what makes a living thing a highly
integrated individual despite the diversity of its constituents. (Like many other
aspects of biological individuality, integration–which we defined as one way of
conceiving cohesion–is a matter of degree.) According to the immunological
account proposed here, every living thing is a chimera, a heterogeneous and
mixed entity composed of genetically diverse components (Pradeu 2010; Dupré
2010 ). On the one hand, it is a composite entity, comprising biotic elements that
originate both from the organism and from outside the organism (bacteria,
viruses, fungi, and so on). On the other hand, it is a very special composite entity,
exhibiting a high degree of integration, with well-delineated boundaries, tight
interactions, and strong cooperation between its components. Because of its three
key activities offiltering over entry,filtering over presence, and promotion of
cooperation, the immune system is essential in determining the constituents and
boundaries of the living thing and therefore in turning a set of heterogeneous
components into an integrated individual. In other words, the immune system
exerts a major“epluribusunum”activity: it constantly turns a plurality of
elements that are diverse and of various origins into a cohesive unit. From this
point of view, the immune system, along with other (and closely connected)
“policing”mechanisms (such as apoptosis) and systems pertaining to adhesion
and intercellular communication, participates in the“glue”of life, the sticking
together of diverse elements that eventually constitute a physiological unit. (For
a similar claim centered on the specific example of sponges, see (Müller 2003).)
24 Elements in the Philosophy of Biology