Disease risk attributed to co-inheritance of KIR2DS1 and HLA-C2 has been previously observed in two independent AS cohorts [59, 65], as well as in psoriasis vulgaris [93, 94] and psoriatic arthritis [18], whereas it is associated with protection from Hodgkin’s lymphoma [95] and the anti-leukemic activity of alloreactive NK cells in a HLA-C2 context [96]. Here, C2 is linked to psoriatic arthritis.