Indirect Coombs Test

Indirect Coombs Test (Indirect Antiglobulin Test)

The Indirect Coombs Test is used to detect antibodies in the bloodstream that are produced against red blood cells. One of the primary reasons for the formation of red blood cell antibodies in our blood is exposure to foreign red blood cells during blood transfusion or pregnancy.

Normal Presence of Antigens on Red Blood Cells

• Red blood cells naturally have specific antigens on their surface, determined by maternal and paternal genetics.
• The main antigens on red blood cells are A and B, which determine blood groups in the ABO system.
• Another antigen, D, determines the Rh factor:
  • If the D antigen is present, the blood is Rh-positive.
  • If absent, the blood is Rh-negative.
• In addition to these, there are numerous other red blood cell antigens, such as the less common Kell, Lewis, and Kidd blood groups.

Reasons for Developing Antibodies Against Red Blood Cells

1. Following a Blood Transfusion:

   • Antibodies against red blood cell antigens A and B are naturally produced without exposure to these antigens. Before transfusions, ABO and Rh blood grouping is performed to avoid serious transfusion reactions.
   • Your ABO group and Rh factor must match those of the donor to prevent your antibodies from reacting with the donor's red blood cells.
   • However, other red blood cell antigens (e.g., Kell or Kidd) may also trigger antibody production after exposure during a transfusion.
   • People who have received multiple transfusions are more likely to develop antibodies against red blood cells due to exposure to a wider variety of antigens.

2. Mother-Child Blood Group Incompatibility:

   • Antigens absent in the mother's red blood cells may be genetically passed to the child from the father.
   • During pregnancy or delivery, fetal red blood cells may enter the mother’s bloodstream through the placenta.
   • The mother’s immune system recognizes these foreign antigens and produces antibodies against them.
   • This condition is typically not seen during the first pregnancy but occurs in subsequent pregnancies. Maternal antibodies can cross the placenta, attach to the baby’s red blood cells, and cause hemolytic disease of the fetus/newborn by hemolyzing them.
   • The Indirect Coombs Test can detect whether the mother has developed antibodies against red blood cells.

Antibody Formation Process:

• Antibodies are initially produced after the first exposure to a foreign red blood cell antigen during a blood transfusion or pregnancy.
• During the first exposure, there is insufficient time for the immune system to destroy foreign red blood cells.
• In subsequent transfusions or pregnancies, the immune system is primed and produces enough antibodies to bind to and destroy transfused red blood cells or fetal red blood cells, leading to hemolysis.