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