Type III hypersensitivity occurs when antigens and antibodies are present in roughly equal amounts, causing extensive cross-linking.

It is characterized by soluble antigens that are not bound to cell surfaces (which is the case in type II hypersensitivity). When these antigens bind antibodies, large immune complexes form^[1] that cannot be cleared. They deposit in vessel walls and induce an inflammatory response, and can cause tissue damage wherever they precipitate. The reaction can take hours, days, or even weeks to develop.