An allergy is commonly understood to be an overreaction of the immune system to harmless substances that are misrecognised as foreign. This concept of allergy as an abnormal, misdirected immune response—a biological fault—stems from the idea that the immune system is an inherently defensive operation designed to protect the individual through an innate capacity to discriminate between the benign and toxic, or self and nonself. However, this definition of allergy represents a radical departure from its original formulation. Literally meaning ‘altered (...) reactivity’, the term was coined in 1906 by Austrian paediatrician Clemens von Pirquet, to describe the fundamentally mutable nature of the immune response.This paper argues that the conventional interpretation of allergy-as-pathology derives from specific concepts of ‘organism’, ‘response’, and ‘normal’ immune function that have—for over a century—governed the perception and study of immune phenomena within immunology. Through an examination of Louis Pasteur’s conceptualisation of the host body/microorganism relationship, I argue that immunology is founded on a view of the organism as a discrete, autonomous entity, and on a concomitant notion of the immune response as essentially reactive. Revisiting the concept of ‘altered reactivity’, this paper points to the fact that allergy was initially posited as a general theory of immune responsiveness and, importantly, one that poses a significant challenge to orthodox notions of immunopathology. It suggests that Pirquet’s unique view of immune responsiveness presents an account of organismic or biological identity that encapsulates, rather than reduces, its ecological complexity. (shrink)