Blood is categorized into different groups based on the presence or absence of specific inherited protein molecules on the surface of red blood cells. These molecules, known as antigens, trigger an immune response if they are foreign to the body. Plasma also contains natural antibodies that attack opposing antigens. Understanding these markers prevents fatal immune reactions during medical procedures.
The ABO system classifies blood into four major types based on A and B antigens.
The Rhesus (Rh) system is another crucial protein marker on red blood cells, designated simply as positive or negative. The D antigen is the most clinically significant Rhesus antigen. Individuals possessing this antigen are Rh-positive, while those lacking it are Rh-negative. Unlike ABO antibodies, Rh antibodies do not develop naturally; they only form after an Rh-negative person is exposed to Rh-positive blood through transfusion or pregnancy.
Before any blood product is transfused, laboratory scientists perform blood typing and crossmatching to guarantee safety.
Safe transfusion relies on ensuring that the donor's red blood cells are not attacked by the recipient's plasma antibodies. For red cell concentrates, Type O is the universal donor, and Type AB is the universal recipient. Conversely, for plasma transfusions, the rules are inverted: Type AB plasma is the universal donor because it lacks ABO antibodies, while Type O recipients can only safely receive Type O plasma.
Transfusing incompatible blood leads to an acute hemolytic transfusion reaction, where recipient antibodies destroy donor red blood cells. Symptoms include fever, chills, chest pain, and dark urine due to hemoglobinuria. In obstetrics, hemolytic disease of the newborn occurs if an Rh-negative mother carries an Rh-positive baby, risking sensitization during delivery unless prophylactic anti-D immunoglobulin is administered.