The histological study of the local reaction which follows the intravenous injection of reacting or provocative factor shows exudative inflammation which differs from that due to the preparatory factor principally in the increased general severity and marked damage to blood vessels. The increased severity of reaction which follows the intravenous injection is probably due to concentration of the injurious agent at the site of inflammation determined by the preparatory local injection.

The exudative phenomena caused by the preparatory local injection are edema and infiltration of polymorphonuclear leucocytes and large mononuclear cells. Vascular injury is morphologically demonstrable in only a few of the sections, notably those from periarticular structures. The intravenous injection determines an increase in cellular infiltration, necrosis of exudate, phagocytosis of cell debris, destruction of vascular walls and hemorrhage. Healing is due to granulation, organization and cicatrization.