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:However, neither IL-4 nor IL-13 plays a consistently suppressive role; their effects appear model and/or Leishmania species dependent (22, 23, 27, 30, 31, 49, 53, 56, 57).
Category: