Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:All the above empirical studies examined innate immunity [‘a set of disease-resistance mechanisms that are not specific to a particular pathogen’ (Goldsby et al., 2000)]: a shared feature of vertebrate and invertebrate immunity (Hultmark, 1993; Vilmos & Kurucz, 1998).
Category: