Blood grouping. Laboratory technician pointing to a sample of blood that has agglutinated (stuck together). Blood can be divided into four blood groups; A, B, O and AB. A blood group is decided by the presence of antigens on the surface of red red blood cells (RBCs), which can be A or B, and the antibodies present in blood plasma, which can be a or b. If RBCs come into contact with their antigen's complimentary antibodies (for example, A with a), the reaction will cause the RBCs to agglutinate. The RBCs can not function properly and may block blood vessels. By adding antibodies to blood samples, blood groups can be determined.