To investigate the changes of B and T lymphocyte attenuator ( BTLA ) , reactive oxygen species ( ROS ) , reactive nitrogen species ( RNS ) , malondialdehyde ( MDA ) , superoxide dismutase ( SOD ) , catalase ( CAT ) , total antioxidative capacity ( TAOC ) in the patients with ankylosing spondylitis ( AS ) and the effect of Xinfeng capsule ( XFC ) on them .