The Arthus reaction involves the in situ formation of antigen/antibody complexes after the intradermal injection of an antigen. If the individual has circulating antibody either from passive immunity or because of prior encounter with the antigen, an Arthus reaction may occur. Typical of most mechanisms of the type III hypersensitivity, Arthus manifests as local vasculitis due to deposition of IgG-based immune complexes in dermal blood vessels.
The Arthus phenomenon is characterized by the formation of antigen/antibody complexes following intradermal injection of an antigen. If a person has pre-existing antibodies, either through passive immunity or previous exposure, an Arthus reaction can occur. This type of hypersensitivity reaction typically involves inflammation of blood vessels in the skin due to immune complex deposition.