Arthus Reaction

An Arthus reaction refers to an acute, localized inflammatory response that typically occurs after vaccination. It is classified as a type III hypersensitivity reaction, which is when antigen-antibody clusters, also known as immune complexes, are formed due to an abnormal immune system response. The reaction usually presents at the site of injection after a vaccination (e.g., tetanus-diphtheria booster). In some rare instances, an Arthus reaction may result from multiple, repeated insect bites. 

An Arthus reaction is the result of a localized inflammation of the small vessels (i.e., vasculitis) near the injection or bite site. This response occurs due to the interaction between the injected antigens and the circulating antibodies (e.g., IgG), which are created after repeated exposure to a specific antigen. As a result, immune complexes are formed around and within the blood vessels of the skin. Subsequently, activation of the complement cascade, which is part of the innate immune system, enhances the local inflammation. 

An Arthus reaction is not very common. However, it is possible that many cases of Arthus reactions are not reported, or even diagnosed, due to the generally mild clinical presentations.

The signs and symptoms of Arthus reactions are typically mild and usually develop within 24 hours after the exposure to the triggering factor. Most individuals present with redness, swelling, and pain. It can also subsequently lead to induration (i.e., hardening of the area due to increased fibrous tissue) localized to the affected area. In more severe cases, the redness and swelling can spread throughout the extremities, and localized hemorrhages can be observed near the site of injection. Rarely, necrosis and ulcerations of the skin have been reported in very severe cases. The signs and symptoms may persist from a week to a few months, however, they typically leave no scars after healing. 

The treatment is based on the severity of the reaction. Most mild cases resolve on their own and do not require medical intervention. In more severe cases, steroids and antihistamines (i.e., medication used to alleviate allergy symptoms) can be used to restrict the inflammatory response.