functional ascription in the case of the immune system. It increasingly appears
that immune systems have evolved under different selective pressures, which
include the necessity to defend the organism against pathogens but are much
more diverse than that. Natural selection has favored immune systems efficient
at simultaneously defending, constructing, repairing, cleaning up, and main-
taining the organism. Many trade-offs exist among these various selective
pressures. For example, so-called type 2 immunity (essential for repair, but
also in response to parasites) is partly at odds with type 1 immunity (which
responds to intracellular abnormalities, including viruses and intracellular bac-
teria), as the activation of one weakens the efficacy of the other. Current
immune systems function by balancing these different dimensions and reaching
a complex equilibrium between them in the context of present and past envir-
onmental pressures (Eberl 2016).
This discussion also brings up the question of the origins of thefirst immune
systems. The capacity to regulate the various processes of defense, development,
repair, clearance of debris, and maintenance of homeostasis is probably as ancient
as life itself. If so, then all living things that currently exist or have existed in the
past have an immune system. The key question, therefore, becomes how replicat-
ing molecules started to acquire, a few billions of years ago, the capacity to
coalesce into groups, stick together, and maintain, repair, and defend themselves
at this emerging group level. Much later, the same question was raised for the
DefenceImmune systemRepairClearanceDevelopmentMaintenance of
tissue homeostasisFigure 2.2 Extended immunity: overview of the various, partly
overlapping, activities of the immune system.The immune system not only
defends the organism against different potential threats but also constructs,
repairs, cleans up the organism, and maintains tissue homeostasis, among other
activities.
Philosophy of Immunology 11