(Billingham et al. 1953). For Burnet, therefore, the self is acquired, not innate:
the organism acquires at an early ontogenetic stage the capacity to recognize its
own constituents and to avoid their destruction (Burnet and Fenner 1949;
Burnet 1969). But Burnet argues that, in nature, contrary to what happens in
these experimental settings, the immunological self reflects the genetically
endogenous components of the organism because the repertoire of immune
receptors is constituted on the basis of these endogenous elements present in the
body (Burnet 1962). Burnet considers several obvious challenges to the self–
nonself framework, including autoimmune diseases and foeto-maternal toler-
ance as well as other forms of immunological tolerance (that is, the absence of
destruction of a foreign entity by the immune system). But he treats them as
exceptionsto the general rule and considers that these exceptions must be
explained via specific mechanisms (pathological mechanisms in the case of
autoimmune diseases, and particular provisory mechanisms in the case of foeto-
maternal tolerance) (Burnet 1969). All these reflections constituted the founda-
tions of a fruitful theoretical and conceptual framework about the self and
nonself, which was developed by Burnet over more than three decades, and
which combined in an innovative way molecular, cellular, ecological,
and medical considerations (Burnet 1940, 1969, pp. 309–310). Burnet shared
the 1960 Nobel Prize in Physiology/Medicine with Medawar for what was
essentially a contribution of a conceptual and hypothetical nature, as acknowl-
edged by Burnet himself (Burnet 1960, p. 700).
From a theoretical viewpoint, what Burnet sees as his main contribution to
immunology is not primarily the self–nonself theory but rather the“clonal
selection theory.” The clonal selection theory goes against instructionist
approaches to antibody formation (especially that of Linus Pauling) by stating
that, when an antigen penetrates into the organism, immune cells bearing
receptors specific for this antigen undergo selection and are subsequently
responsible for the elimination of that antigen (Burnet 1959, p. 54). Burnet
therefore proposes the adoption of a Darwinian framework at the cell level
within the multicellular organism (Burnet 1959, p. 64)–an idea that had been
suggested before him but that Burnet framed in a much more precise way
(Schaffner 1992; Silverstein 2002).
Nonetheless, the clonal selection theory and the self–nonself theory are
intimately connected, so much so that Burnet generally argues for them in
parallel (e.g., (Burnet 1959, 1969)). The phenomena that the clonal selection
theory seeks to explain are immune recognition of the antigen, immune toler-
ance, and the acquisition of“self-knowledge”by the organism. To account for
these phenomena, Burnet proposes the existence of two selective processes: one
occurs in adult life, when an antigen enters into the body, as we just saw; but
16 Elements in the Philosophy of Biology