Immune Complex Test: In protecting the body from foreign invaders (antigens), the immune system produces proteins called antibodies. Immune complex reactions result when large numbers of antigen-antibody complexes form in the body. The immune complex test is used to identify circulating immune complexes in the blood. The process usually involves examination of a tissue obtained by a biopsy. Specific staining techniques are used to identify specific antibodies. To determine the level of complexes still in the circulatory system serum assays are also obtained from blood samples.