Researchers now regard interferon as not a single substance, but it is rather a biological family of complex molecules that play an important, though not entirely defined, role in the immune system.
                    
                
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                Aas not a single substance, but it is rather a biological family of complex molecules that play
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                Bas not a single substance but as a biological family of complex molecules playing
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                Cnot as a single substance but as a biological family of complex molecules that play
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                Dnot to be a single substance but rather a biological family of complex molecules playing
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                Enot as a single substance but instead as being a biological family of complex molecules that play
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