Spondylarthropathies: Heterogeneous group of arthritic diseases sharing clinical and radiologic features. They are associated with the HLA-B27 ANTIGEN and some with a triggering infection. Most involve the axial joints in the SPINE, particularly the SACROILIAC JOINT, but can also involve asymmetric peripheral joints. Subsets include ANKYLOSING SPONDYLITIS; REACTIVE ARTHRITIS; PSORIATIC ARTHRITIS; and others.HLA-B39 Antigen: A specific HLA-B surface antigen subtype. Members of this subtype contain alpha chains that are encoded by the HLA-B*39 allele family.HLA-B27 Antigen: A specific HLA-B surface antigen subtype. Members of this subtype contain alpha chains that are encoded by the HLA-B*27 allele family.Spondylitis, Ankylosing: A chronic inflammatory condition affecting the axial joints, such as the SACROILIAC JOINT and other intervertebral or costovertebral joints. It occurs predominantly in young males and is characterized by pain and stiffness of joints (ANKYLOSIS) with inflammation at tendon insertions.