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Is "known" an adjective while "unknown" is a noun here?

The known unknowns of T cell immunity to COVID-19

Abstract: Tremendous progress has been made in understanding the role of T cell immunity in acute and convalescent COVID-19 infection. Here we shed light on the “known unknowns” of pre-existing and acquired T cell responses in relation to acute and convalescent SARS-CoV-2 infection.

Source: ScienceImmunology

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Yes, "known" is an adjective here and "unknown" is a noun.

Known and unknown unknowns have been popularly discussed in this way since Donald Rumsfeld famously used the terms in 2002. Apparently these phrases existed before that, but they were not in popular use. Today, a great many English speakers are familiar with them, even if they don't know the origin or how recent it is.

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