The direct antiglobulin test (DAT) looks for antibodies attached to red blood cells (RBCs) circulating in the bloodstream. RBCs have structures on their surfaces called antigens. Each person has their own individual set of RBC antigens, determined by inheritance from their parents. The major antigens or surface identifiers on human RBCs are the O, A, and B antigens, and a person's blood is grouped into an A, B, AB, or O blood type according to the presence or absence of these antigens