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:Even though several studies have found no difference in reaction rates between membranes that readily activate complement and those that do not, a subset of patients (those with atopia and/or eosinophilia) seems to be more susceptible to dialysis reactions and experiencing recurrent symptoms [6].
Category: