This is a type III hypersensitivity reaction mediated by immune complex deposits. Immune complexes are antigen-antibody (commonly IgG) complexes that are soluble and prone to deposition in multiple organs. Once immune complexes are deposited in an organ, neutrophils and macrophages will then attack the organ causing organ damage and eventually failure. Type III hypersensitivity reactions are characteristic in SLE and other autoimmune diseases such as rheumatoid arthritis, etc.
Other types are type I hypersensitivity which are mediated by mast cells and histamine with the involvement of IgE and this commonly happens in allergic reactions. Type II hypersensitivity is cytotoxic hypersensitivity wherein antibodies directly attack organs (not forming immune complexes). Type IV hypersensitivity (or cell-mediated toxicity) involves T-lymphocytes. This is a delayed type of hypersensitivity exemplified by reactions from <em>M. tuberculosis</em> bacilli in tuberculous disease.
