The term “Type and Crossmatch” refers to a mandatory two-part laboratory procedure performed before a patient receives a red blood cell transfusion. This process is designed to ensure the blood product is fully compatible with the recipient, which is a foundational safety measure in transfusion medicine. The procedure is the final verification that the donor blood will not be mistakenly recognized as a threat by the patient’s immune system. By carefully testing for specific markers, the procedure drastically reduces the risk of a severe, life-threatening reaction that incompatible blood can cause.
The First Step: Blood Typing (The “Type”)
Blood typing, the initial stage, identifies the patient’s major blood group antigens and their Rhesus (Rh) status. This process focuses on the ABO system, which classifies blood into four main types—A, B, AB, and O—based on the antigens present on the surface of the red blood cells. Antigens are protein markers that can trigger an immune response if they are foreign to the body.
A person with Type A blood has A antigens and naturally occurring anti-B antibodies in their plasma, while a person with Type B blood has B antigens and anti-A antibodies. Type O blood has neither A nor B antigens but possesses both anti-A and anti-B antibodies, which is why it is the universal donor of red blood cells. Conversely, Type AB blood has both A and B antigens but no anti-A or anti-B antibodies, making it the universal recipient.
Beyond the ABO system, the Rh factor is the second most important classification, determining if blood is positive (+) or negative (-). This factor is determined by the presence or absence of the RhD antigen on the red blood cells. Rh-negative individuals do not naturally have the anti-RhD antibody, but they can produce it if exposed to Rh-positive blood, which is a particular concern in pregnancy. The typing process also includes a screen for unexpected antibodies that may have developed from previous transfusions or pregnancies.
The Final Check: Crossmatching (The “Cross”)
The crossmatch is the final, hands-on compatibility test that serves as a trial transfusion conducted in a laboratory setting. The primary method, known as the major crossmatch, involves mixing a sample of the patient’s plasma or serum with a sample of the donor’s red blood cells.
This mixture is then observed for a reaction, typically involving an incubation period followed by centrifugation. If the patient’s plasma contains antibodies that recognize and attack antigens on the donor’s red blood cells, they will bind to the cells and cause them to clump together. This visible clumping, or agglutination, signifies incompatibility, and the specific unit of blood is then ruled out for transfusion.
A process called the electronic crossmatch can be used for patients who have a negative antibody screen and no history of clinically significant antibodies. This method bypasses the physical mixing of samples and relies on a validated computer system to verify ABO and Rh compatibility between the patient’s recorded blood type and the donor unit. Regardless of the method used, the crossmatch is the last safeguard to confirm the blood unit is safe for the patient before it is released for transfusion.
What Happens When Compatibility Fails?
If incompatible blood is transfused, even a small amount, it can trigger a severe and potentially fatal immune response called an Acute Hemolytic Transfusion Reaction (AHTR). This reaction typically occurs when an ABO mismatch allows the patient’s pre-existing antibodies to rapidly destroy the transfused red blood cells. This destruction, known as intravascular hemolysis, releases free hemoglobin into the bloodstream.
The sudden release of hemoglobin can lead to severe organ damage, especially to the kidneys, and trigger a widespread clotting cascade known as disseminated intravascular coagulation (DIC). Symptoms often begin abruptly, sometimes within minutes of the transfusion starting, and may include high fever, chills, severe back or flank pain, and a drop in blood pressure leading to shock.
The type and crossmatch procedure is designed to catch errors, such as mislabeling a sample or administering the wrong unit, before they reach the patient. This rigorous, multi-step testing process is a requirement in blood banking, specifically designed to prevent this rare but catastrophic outcome.