An immunological test aimed at detecting the presence of free and unbound IgG antibodies (and sometimes IgM) in the patient’s serum or plasma. These antibodies are known as "irregular antibodies" because they are not naturally present in everyone. The test is performed by mixing the patient’s serum with red blood cells of known antigens, then adding anti-human serum (Coombs serum). If the patient’s serum contains antibodies against the tested cell antigens, they will bind to them, and adding Coombs serum will cause agglutination, indicating a positive result.