Immune complex mediated hypersensitivity
Type III hypersensitivity reactions are also termed immune complex reactions. Complexes of antigen and antibody form in the circulation and are then deposited in susceptible tissues; they may also form directly in the tissue. The latter mechanism is termed the Arthus reaction, and is typically seen with repeated insect stings, where a red swollen lesion develops after a sting. The tissue damaging mechanisms are similar to those described for the antigen-antibody complexes that form in type II responses. The response times of types II and III hypersensitivity reactions are slower than that of type I reactions; they typically develop 3–6 h after exposure to antigen. The response can also become chronic, particularly in autoimmune reactions, where antigen persists. The clinical manifestations of type III hypersensitivity reactions relate to the tissue deposition, for example vasculitic (skin), serum sickness (systemic), nephritis (kidneys) and extrinsic allergic alveolitis (lungs).