Coombs Test - Indirect

The red blood cells have specific antigen markers present on their surface. These marker antigens are glycoproteins and help the body to recognize its own type of Red Blood Cells (RBCs). Antibodies are naturally produced against incompatible RBC antigens when there is exposure to incompatible red blood cells. Three commonest antigen markers found on RBCs are A, B, and Rh, and the presence or absence of these antigen markers is used to determine the blood group of a person.

Blood transfusion between incompatible blood types causes an antigen-antibody reaction which causes the RBCs to clump together (agglutination) and this results in their destruction. Hence blood groups of donor and recipient must match to ensure the success of the blood transfusion or organ transplant. Agglutination may also occur due to Rh incompatibility between Rh-negative pregnant woman (who is having Rh antibodies because of previous sensitization) and her Rh-positive child which may cause antibodies in the mother to cross the placenta and attack the RBCs of the foetus. In some autoimmune disorders, antibodies are produced against antigens on the body’s own RBCs. These antibodies may attack the ‘self’ RBCs and cause agglutination.

The Indirect Coombs Test is performed on the principle of the antigen-antibody reaction between incompatible blood groups. The collected blood specimen is treated with a large range of collected or synthetic human RBC antigens and observed under a microscope. If agglutination (clumping) of RBCs is seen upon treatment with a particular antigen, antibodies against that antigen are already present in the blood specimen. Antibodies against RBC antigens A and B occur naturally, while antibodies against other RBC antigens are produced upon exposure to the corresponding antigen. A similar test called the Direct Coombs Test is performed to detect the presence of antibodies (bound to RBCs) against the body’s own RBCs (autoimmune antibodies) that may be the cause of autoimmune hemolytic anemia.