Which of the following represents a Type 3 hypersensitivity reaction?

A Type 3 hypersensitivity reaction is characterized by the formation of immune complexes that can lead to inflammation and tissue damage. The positive TB skin test illustrates this mechanism. In this scenario, when tuberculin is injected into the skin of a previously sensitized individual, the body’s immune system generates antibodies that bind to the antigen, forming immune complexes. These complexes can then deposit in various tissues, causing a localized inflammatory response typically seen within 48 to 72 hours, which is indicative of Type 3 hypersensitivity.

The other options represent different types of hypersensitivity reactions:

• Contact dermatitis is classified as a Type 4 hypersensitivity reaction, mediated by T cells.

• Allergic rhinitis is primarily an example of a Type 1 hypersensitivity reaction, driven by IgE antibodies and causing an immediate allergic response.

• Anaphylaxis is also a Type 1 hypersensitivity reaction, involving a rapid systemic response to an allergen, leading to severe and often life-threatening reactions.

Thus, the choice of a positive TB skin test as a representation of Type 3 hypersensitivity is recognized due to the immune complex mechanism involved in the response.