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:The acute phase response is a very complex reaction, involving local and systemic effects, and may be caused by a range of factors (Heegaard et al., 1998; Carroll et al., 2004; Murata et al., 2004; Dı́az et al., 2005; Grau-Roma et al., 2009, Pomorska-Mól et al., 2011a, b).
Category: