Which condition is described by a local reaction seen in the skin after injection of an antigenic substance?

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The condition characterized by a local reaction in the skin following the injection of an antigenic substance is known as the Arthus reaction. This reaction is a type of localized hypersensitivity response, specifically classified as a type III hypersensitivity, where immune complexes form as a result of the injection. These complexes lead to inflammation, triggered by the deposition of antibody-antigen complexes in the tissues, particularly in the skin.

When an antigen is introduced, it can lead to a localized inflammatory response due to the accumulation of immune complexes and complement activation, resulting in symptoms such as erythema, swelling, and pain at the site of injection. This localized reaction typically occurs hours after the exposure and is often seen in individuals who have high levels of circulating antibodies against the injected antigen.

In contrast, urticaria is characterized by elevated, itchy welts on the skin due to histamine release, anaphylaxis is a systemic, severe allergic reaction that can lead to shock and requires immediate treatment, and allergic contact dermatitis is a delayed hypersensitivity reaction resulting from skin exposure to certain allergens, rather than from injection. These other conditions do not reflect the same mechanism of action or localized response associated with the Arthus reaction.

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