Immune complexes are clusters of interlocking antigens and antibodies. Under normal conditions immune complexes are rapidly removed from the bloodstream by macrophages in the spleen and Kupffer cells in the liver. In some circumstances, however, immune complexes continue to circulate. Eventually they become trapped in the tissues of the kidneys, lung, skin, joints, or blood vessels. Just where they end up probably depends on the nature of the antigen, the class of antibody-IgG, for instance, instead of IgM-and the size of the complex. There they set off reactions that lead to inflammation and tissue damage.

Immune complexes work their damage in many diseases. Sometimes, as is the case with malaria and viral hepatitis, they reflect persistent low-grade infections. Sometimes they arise in response to environmental antigens such as the moldy hay that causes the disease known as farmer's lung. Frequently, immune complexes develop in autoimmune disease, where the continuous production of autoantibodies overloads the immune complex removal system.