R4: HLAB 27In case of an uncertain clinical situation, looking for HLAB 27 is of a diagnostic value, but its absence does not eliminate the diagnosis. HLA B27 is also of prognostic value; its presence is associated with an early disease onset, increased inflammation of the sacroiliac joints on MRI, and the occurrence of uveitis. It is also a predictive factor of the response to anti-TNF in axial forms and in uveitis. Here, TNF is linked to uveitis.