What is the immune phenomenon associated with the Arthus reaction?

• A. Deposition of immune complexes in blood vessels
• B. Release of histamine from mast cells
• C. Removal of antibody-coated RBCs
• D. Tissue destruction by cytotoxic T cells

Answer: (A)

The Arthus reaction is a localized type III hypersensitivity reaction driven by deposition of antigen–antibody immune complexes in the walls of small blood vessels at the site of antigen exposure. When preformed antibodies encounter the intradermal antigen, immune complexes form and settle in vessel walls, activating complement. Complement products recruit and activate neutrophils, whose enzymes and reactive species cause inflammation, edema, and tissue injury at the injection site within a few hours. This mechanism hinges on immune complex deposition, not on histamine release from mast cells (that’s a type I reaction), nor on removal of antibody-coated red cells, nor on cytotoxic T cell–mediated destruction (type IV).