Which type of Hypersensitivity reaction is considered to be immune complex mediated?

In the context of hypersensitivity reactions, immune complex-mediated reactions fall under Type 3 hypersensitivity. This type occurs when antibodies (specifically IgG or IgM) bind to soluble antigens, leading to the formation of immune complexes. These immune complexes can deposit in various tissues throughout the body, particularly in the kidneys, joints, and blood vessels, inciting an inflammatory response.

This inflammatory response is mediated by the complement system and various immune cells, resulting in tissue damage. Conditions commonly associated with Type 3 hypersensitivity include systemic lupus erythematosus (SLE), rheumatoid arthritis, and post-streptococcal glomerulonephritis, where the deposited immune complexes contribute to disease pathology.

In contrast, the other types of hypersensitivity reactions involve different mechanisms: Type 1 is mediated by IgE and is associated with immediate allergic reactions; Type 2 is antibody-mediated and involves direct cell damage; and Type 4 is a delayed-type hypersensitivity characterized by T-cell involvement, without antibody mediation. Understanding the specifics of these pathways helps in determining appropriate clinical management and treatment of the diseases associated with each hypersensitivity type.