Surprisingly, we also found that healthy individuals that carry the PV susceptibility alleles HLA DR4 (DRB1*0402) and/or DR6 (DQB1*0503) (HLA-matched controls) show an upregulation of cytokine and chemokine levels that are on par with those seen in PV patients for certain pro-inflammatory, Th2, and Th17 mediators and IL-8, while healthy controls that did not carry the PV susceptibility alleles (HLA-unmatched controls) express significantly lower levels of these cytokines and chemokines. Here, TNFRSF21 is linked to acquired polycythemia vera.