Fate of antigen after intravenous and intraarticular injection into rabbits.

To determine the manner in which small protein antigens enter the synovium and are localized, horseradish peroxidase (molecular weight 40,000) was injected by intravenous and intraarticular routes. Passage of antigen from capillary lumen to joint space and vice versa occurred primarily across fenestrated vessels. Antigen localized to collagen fibers and lining cells. At low concentrations of antigen, antigen was taken up predominantly by marcophages, whereas at higher concentrations both macrophages and fibroblast-like lining cells accumulated antigen.