People who lack certain blood group antigens on their red cells can form antibodies against these antigens. For example, a person with type A blood will produce antibodies against the B antigen. The ABO blood group antibodies are naturally occurring, meaning that they are found in people who have not been exposed to incompatible blood. Antibodies to most other blood group antigens, including RhD, develop after people are exposed to the antigens through transfusion or pregnancy. Some of these antibodies can bind to incompatible red blood cells and cause them to be destroyed, resulting in transfusion reactions and other complications. Serologic methods for blood typing make use of these antibody-antigen reactions. Reagents containing blood group antibodies, called antisera, are added to suspensions of blood cells. If the relevant antigen is present, the antibodies in the reagent will cause the red blood cells to agglutinate (clump together), which can be identified visually. In addition to identifying the ABO antigens, which is termed forward grouping, routine ABO blood typing also includes identification of the ABO antibodies in the person's plasma. This is called reverse grouping, and it is done to confirm the ABO blood type. In reverse grouping, the person's plasma is added to type A1 and type B red blood cells. The plasma should agglutinate the cells that express antigens that the person lacks, while failing to agglutinate cells that express the same antigens as the patient. If this does not occur, further testing is required. Agglutination is scored from 1+ to 4+ based on the strength of the reaction. In ABO typing, a score of 3+ or 4+ indicates a positive reaction, while a score of 1+ or 2+ is inconclusive and requires further investigation. Blood group antibodies occur in two major forms: Immunoglobulin M (IgM) and Immunoglobulin G (IgG). Antibodies that are predominantly IgM, such as the ABO antibodies, typically cause immediate agglutination of red blood cells at room temperature. Therefore, a person's ABO blood type can be determined by simply adding the red blood cells to the reagent and centrifuging or mixing the sample. RhD typing also typically uses IgM reagents although anti-RhD usually occurs as IgG "in vivo". Antibodies that are predominantly IgG, such as those directed towards antigens of the Duffy and Kidd systems, generally do not cause immediate agglutination because the small size of the IgG antibody prevents formation of a lattice structure.