The passive Arthus arthritis in the guinea pig provides an experimental model for studying reactions comparable to those which occur in rheumatic fever. The joint swelling in the Arthus reaction is followed by tissue injury which is reflected 24 hours later by a rise in the DPA level. The height of this rise is determined by the degree of injury, which in turn is determined both by the strength of the antibody injected and also by the number of antigen depots created.
The swelling reaction and the DPA rise can be suppressed by sodium salicylate, cortisone, or splenin A. The amount of drug required to block a DPA rise (DPA unit) is approximately tenfold the amount required to inhibit the joint swelling by 50 per cent (Arthus unit). Even when the Arthus arthritis has completed the swelling phase, treatment with appropriate amounts of these drugs suppress a rise in DPA.
It was pointed out that the DPA rise appears to be associated with the tissue injury that follows joint swelling; that the degree of a DPA rise is determined by the severity of the inflammatory process; and that the amount of an anti-inflammatory agent required to suppress a DPA rise depends on the intensity of the reaction.