What type of hypersensitivity reaction involves the formation of immune complexes?

The type of hypersensitivity reaction that involves the formation of immune complexes is Type 3 hypersensitivity. In this reaction, antibodies, typically of the IgG or IgM class, combine with soluble antigens to form immune complexes. These complexes can deposit in various tissues, such as the skin, kidneys, and joints, leading to inflammation and tissue damage.

The pathogenic mechanism of Type 3 hypersensitivity involves the binding of these immune complexes to Fc receptors on cells, triggering an inflammatory response through the activation of complement and recruiting inflammatory cells, particularly neutrophils. This response can manifest in conditions such as systemic lupus erythematosus and rheumatoid arthritis, showcasing the detrimental effects of immune complex deposition on the body's tissues.

Type 1 hypersensitivity is associated with immediate reactions mediated by IgE antibodies and mast cells, while Type 2 involves direct antibody-mediated cytotoxicity against cells, often involving IgG or IgM. Type 4 hypersensitivity, on the other hand, is delayed and mediated by T cells rather than antibodies. Thus, the unique involvement of immune complexes in Type 3 hypersensitivity is what makes this response distinct and relevant.