How can antigen-antibody complexes cause tissue injury?

Antigen-antibody complexes, also known as immune complexes, cause tissue injury primarily through a process called Type III hypersensitivity. This occurs when these complexes are deposited in various tissues, such as blood vessels, kidneys, joints, and skin, rather than being cleared by the body's natural mechanisms.1

The injury is mediated by the following mechanisms:

• Activation of Complement: When deposited in tissues, these complexes activate the complement system. This cascade increases blood vessel permeability, releases histamine, and attracts inflammatory cells like granulocytes and macrophages to the site.21
• Inflammatory Response: The influx of these cells leads to an inflammatory response. The activated cells and complement components release tissue-damaging substances, including enzymes that destroy local tissue, and cytokines like interleukin-1, which can induce fever.1
• Vascular Damage: In conditions such as necrotizing vasculitis, the deposition of these complexes in the walls of small blood vessels leads to inflammation and necrosis, which can result in structural damage like aneurysms or impaired organ function.34

This process is a central feature in several autoimmune diseases, including systemic lupus erythematosus and rheumatoid arthritis.31