The ABO blood group system was the first to be discovered because anti-A and anti-B are mainly of the IgM immunoglobulin class and cause visible agglutination of group A or B red cells in laboratory mixing tests. Antibodies to ABO antigens are naturally occurring and are found in everyone after the first 3 months of life. Many other blood group antibodies, such as those against the Rh antigens, are smaller IgG molecules and do not directly cause agglutination of red cells. These ‘incomplete antibodies’ can be detected by the antiglobulin test (Coombs’ test) using antibodies to human IgG, IgM or complement components (‘antiglobulin’) raised in laboratory animals. The direct antiglobulin test (DAT) is used to detect antibodies present on circulating red cells, as in autoimmune haemolytic anaemia or after mismatch blood transfusion. Blood group antibodies in plasma are demonstrated by the indirect antiglobulin test (IAT). Nearly all clinically significant red cell antibodies can be detected by an IAT antibody screen carried out at 37°C (see section 2.7).