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:Although several antigens including CD163 and CD204 are known to be markers of M2 macrophages, only a few studies have described macrophage activation phenotypes (such as M1 and M2) in human cancer tissues [39,40].
Category: