Arthus reaction is a localized form of which hypersensitivity?

Arthus reaction is a localized form of which hypersensitivity?

• A. Type I
• B. Type II
• C. Type III
• D. Type IV

Answer: (C)

The Arthus reaction is a localized immune complex–mediated inflammation, a type III hypersensitivity. It happens when preexisting antibodies against an antigen (typically IgG) form immune complexes in the vessel walls of the skin after a local antigen exposure. These immune complexes activate complement, generating C3a and C5a that recruit and activate neutrophils, leading to increased vascular permeability, edema, and necrotizing vasculitis at the site. This mechanism—immune complex deposition with complement activation causing local tissue inflammation—fits Type III hypersensitivity, rather than the other types which involve IgE-mediated mast cell activation (Type I), antibody-dependent cell cytotoxicity against surface antigens (Type II), or T-cell–mediated delayed responses (Type IV). The systemic version of this mechanism is serum sickness, a broader Type III reaction.

The Arthus reaction is a localized immune complex–mediated inflammation, a type III hypersensitivity. It happens when preexisting antibodies against an antigen (typically IgG) form immune complexes in the vessel walls of the skin after a local antigen exposure. These immune complexes activate complement, generating C3a and C5a that recruit and activate neutrophils, leading to increased vascular permeability, edema, and necrotizing vasculitis at the site. This mechanism—immune complex deposition with complement activation causing local tissue inflammation—fits Type III hypersensitivity, rather than the other types which involve IgE-mediated mast cell activation (Type I), antibody-dependent cell cytotoxicity against surface antigens (Type II), or T-cell–mediated delayed responses (Type IV). The systemic version of this mechanism is serum sickness, a broader Type III reaction.