This reasoning allows us to propose an immunological definition of the
organism, understood as a physiological individual (i.e., a cohesive whole,
functioning collectively as a regulated unit that persists through time).
According to this definition, an organism is a continuously changing, function-
ally integrated whole made up of heterogeneous constituents (including many
microbes) that are locally interconnected by strong biochemical interactions
and controlled at a systemic level by immunological interactions (Pradeu 2010).
Everything that is actively interacting locally and tolerated by the immune
system is part of the physiological individual.
This means that immunological processes can help us identify and delineate
physiological individuals not via intuition and common sense (as has often been
a concern with physiological definitions) but on the basis of a clearly formulated
and scientifically grounded criterion (following the argument of (Hull 1992)).
The boundaries of the immunologically delineated living thing may be a matter
of degree and vary depending on the context, but the criterion itself is precise
and clear. For example, there are uncertainties and debates over what counts as
a biological individual in the case of a colonial entity such as the ascidian
Botryllus schlosseri(Figure 3.2): each cell, each zoid, the whole colony, or
perhaps all of them together (Buss 1999)? If we adopt the immunological
perspective suggested here, then the whole colony should be recognized as
the physiological individual because it is at the level of the colony that immu-
nological processes occur such as histocompatibility-based fusion/rejection
responses when two colonies meet (De Tomaso et al. 2005). Similarly, there
are many discussions about whether or not the microbiota is part of the
individual it inhabits (e.g., (Gilbert et al. 2012)). Our immunological account
tells us that all (and only) the microbes that are immunologically tolerated are
part of the physiological individual.
Crucially, organismality here is a product, not a given. Indeed, my claim is
notthat the components of an already identified organism are tolerated by its
immune system. Instead, I propose that it is the identification of a local con-
centration of biochemical and immunological processes taken together that tells
us how to pick out organisms (understood as physiological individuals) in
nature on the basis of the capacity of the immune system to determine what
the constituents and boundaries of a living thing are. We start with an unin-
dividuated living mass (Botryllus schlosseri, for example), but the identification
of immunological processes allows us to single out and delineate quite precisely
a physiological individual.
Biological individuation, as we saw, can be realized at different levels. An
interesting aspect of immunological individuation is that it also can occur at
different levels. Multicellular organisms exhibit what can be called amultilevel
Philosophy of Immunology 25