Immune complex

An immune complex, sometimes called an antigen-antibody complex or antigen-bound antibody, is a molecule formed from the binding of multiple antigens to antibodies.^[1] The bound antigen and antibody act as a unitary object, effectively an antigen of its own with a specific epitope. After an antigen-antibody reaction, the immune complexes can be subject to any of a number of responses, including complement deposition, opsonization,^[2] phagocytosis, or processing by proteases. Red blood cells carrying CR1-receptors on their surface may bind C3b-coated immune complexes and transport them to phagocytes, mostly in liver and spleen, and return to the general circulation.

The ratio of antigen to antibody determines size and shape of immune complex.^[3] This, in turn, determines the effect of the immune complex. Many innate immune cells have FcRs, which are membrane-bound receptors that bind the constant regions of antibodies. Most FcRs on innate immune cells have low affinity for a singular antibody, and instead need to bind to an immune complex containing multiple antibodies in order to begin their intracellular signaling pathway and pass along a message from outside to inside of the cell.^[3] Additionally, the grouping and binding together of multiple immune complexes allows for an increase in the avidity, or strength of binding, of the FcRs. This allows innate immune cells to get multiple inputs at once and prevents them from being activated early.^[3]

Immune complexes may themselves cause illness when they are deposited in organs, for example, in certain forms of vasculitis. This is the third form of hypersensitivity in the Gell-Coombs classification, called type III hypersensitivity.^[4] Such hypersensitivity progressing to disease states produces the immune complex diseases.

Immune complex deposition is a prominent feature of several autoimmune diseases, including rheumatoid arthritis, scleroderma and Sjögren's syndrome.^[5][6] An inability to degrade immune complexes in the lysosome and subsequent accumulation on the surface of immune cells has been associated with systemic lupus erythematosus.^[7][8]