Determining a person’s blood type is a foundational procedure in health care, essential for compatibility, especially in transfusions. Blood typing relies on identifying specific marker molecules, known as antigens, present on the surface of red blood cells. The process involves collecting a blood sample and observing how it reacts to specific laboratory reagents. Understanding the results requires knowledge of these markers and the two main classification systems: ABO and Rhesus (Rh).
The Core Components of Blood Typing
Antigens are molecules on the surface of red blood cells that signal the cell’s identity. The ABO system is defined by the presence of the A antigen, the B antigen, both, or neither. Antibodies are specialized proteins found in the plasma that recognize and attack foreign antigens. A person’s plasma naturally contains antibodies against the A or B antigens they do not possess. For instance, a person with only the A antigen has anti-B antibodies, while a person with both A and B antigens has neither. The Rh system focuses on a single protein called the D antigen.
Interpreting ABO Test Results
Blood typing uses agglutination, or clumping, as the indicator for a positive result. This visible reaction occurs when an antibody encounters its matching antigen, causing red blood cells to stick together. The ABO test uses two complementary steps, forward and reverse typing, whose results must agree for the final determination.
Forward typing identifies the antigens on the patient’s red blood cells. The cells are mixed with known anti-A and anti-B antibodies. Clumping with anti-A means the A antigen is present, and clumping with anti-B means the B antigen is present. Clumping with both reagents indicates blood type AB, while no clumping with either designates the type as O.
Reverse typing confirms this result by checking for antibodies in the patient’s plasma. The plasma is mixed with commercial red blood cells known to have either the A or B antigen. If the plasma causes clumping with A cells, it contains anti-A antibodies, characteristic of type B or O blood. Conversely, clumping with B cells shows the presence of anti-B antibodies, typical of type A or O blood.
Determining the Rh Factor and Final Notation
After the ABO type is established, the Rhesus (Rh) status is determined based on the presence or absence of the D antigen. This test is performed by mixing the patient’s red blood cells with a reagent containing the anti-D antibody. If agglutination occurs, the D antigen is present, and the person is designated as Rh positive.
If no clumping occurs in the anti-D test, the D antigen is considered absent, and the person is designated as Rh negative. This Rh status is combined with the ABO result to create the final, standard blood type notation. For example, a person whose cells reacted with anti-A but not anti-B, and positively for the D antigen, would be classified as A positive (A+).
Conversely, an individual who did not react with anti-A or anti-B, and showed no clumping with anti-D, would be reported as O negative (O-). The final report includes both the letter designation from the ABO system and the plus or minus sign from the Rh system. The eight main blood types are A+, A-, B+, B-, AB+, AB-, O+, and O-.
Why Your Blood Type Matters Clinically
Knowing your complete blood type is necessary for safety in medical procedures, particularly blood transfusions. If a patient receives blood containing antigens their body lacks, the recipient’s pre-existing antibodies will attack the donor cells, leading to a severe reaction. Type O negative blood is called the universal donor because its red cells lack A, B, and Rh antigens, making it compatible for transfusion into any recipient in an emergency.
The Rh factor holds specific importance during pregnancy if an Rh-negative mother is carrying an Rh-positive baby. If the baby’s Rh-positive red cells enter the mother’s bloodstream, her immune system may produce anti-D antibodies. These antibodies can cross the placenta in a subsequent pregnancy and attack the red blood cells of a future Rh-positive fetus. To prevent this sensitization, Rh-negative mothers are routinely given an injection of Rh immune globulin (Rhogam), which prevents the formation of these antibodies.