Spondylarthritis: Inflammation of the joints of the SPINE, the intervertebral articulations.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.Rats, Transgenic: Laboratory rats that have been produced from a genetically manipulated rat EGG or rat EMBRYO, MAMMALIAN. They contain genes from another species.Sacroiliac Joint: The immovable joint formed by the lateral surfaces of the SACRUM and ILIUM.Sacroiliitis: Inflammation of the SACROILIAC JOINT. It is characterized by lower back pain, especially upon walking, fever, UVEITIS; PSORIASIS; and decreased range of motion. Many factors are associated with and cause sacroiliitis including infection; injury to spine, lower back, and pelvis; DEGENERATIVE ARTHRITIS; and pregnancy.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.Synovial Membrane: The inner membrane of a joint capsule surrounding a freely movable joint. It is loosely attached to the external fibrous capsule and secretes SYNOVIAL FLUID.Arthritis, Rheumatoid: A chronic systemic disease, primarily of the joints, marked by inflammatory changes in the synovial membranes and articular structures, widespread fibrinoid degeneration of the collagen fibers in mesenchymal tissues, and by atrophy and rarefaction of bony structures. Etiology is unknown, but autoimmune mechanisms have been implicated.