Robbins basic pathology 9th edition page no 115 Type 3 hypersensitivity includes immune complex diseases. Antigen antibody(immune) complexes that are formed in circulation may deposit in blood vessels leads to complement activation and acute inflammation. Antigen-antibody (immune) complexes that are formed in the circulation may deposit in blood vessels, leading to complement activation and acute inflammation. The antigens in these com- plexes may be exogenous antigens, such as microbial pro- teins, or endogenous antigens, such as nucleoproteins. The mere formation of immune complexes does not equate with hypersensitivity disease; small amounts of antigen- antibody complexes may be produced during normal immune responses and are usually phagocytosed and destroyed. It is only when these complexes are produced in large amounts, persist, and are deposited in tissues that they are pathogenic. Pathogenic immune complexes may form in the circulation and subsequently deposit in blood vessels, or the complexes may form at sites where antigen has been planted (in situ immune complexes). Immune complex-mediated injury is systemic when complexes are formed in the circulation and are deposited in several organs, or it may be localized to paicular organs (e.g., kidneys, joints, or skin) if the complexes are formed and deposited in a specific site. The mechanism of tissue injury is the same regardless of the pattern of distribution; however, the sequence of events and the conditions leading to the formation of systemic and local immune complexes are different and are considered separately in the following descriptions. Immune complex diseases are some of the most common immunologic diseases (Table 4-4).