Antigen-induced arthritis: an immunohistologic study of articular tissue and synovial fluid using the horseradish peroxidase technique.

Articular tissue and synovial fluid from rabbits with antigen-induced arthritis were stained for localization of immunoglobulin (Ig) and egg albumin (EA) using the immunoperoxidase technique. Discrete granules containing Ig and EA were initially observed within vascular synovial tissue with later occurrence in synovial fluid and dense connective tissue. Homogeneous staining for Ig was not always associated with the presence of antigen and, therefore, cannot be used alone as evidence of the presence of immune complexes. Discrete granules of Ig and EA are formed in dense connective tissue during the period of maximal inflammation and persist for at least 30 days. These granules presumably represent precipitated immune complexes of Ig and EA but can be stained for Ig alone longer than for EA. Insoluble, particulate immune complexes of Ig and EA appeared in synovial fluid within 2 h after injection and remained at least 24 h. These were identical in appearance to the Ig precipitates occurring in synovial fluids from human rheumatoid arthritis.