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:These findings are in concordance with previously published studies suggesting a negative impact of Af sensitization in CF.15‚Äö√Ñ√¨17 Af sensitization likely facilitates CF-ABPA and multiple factors, including degree of fungal exposure, mucus viscosity, immune status, atopy and age may also have interdependent roles.12,32,39 Other factors, such as HLA-DR/HLA-DQ subtypes and the presence of single nucleotide polymorphisms
 14
 in the IL-4 binding site of IL-4 receptor alpha have also been described to associate with development of ABPA.40‚Äö√Ñ√¨42 Longitudinal studies are needed to confirm that Af sensitization routinely precedes ABPA.
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