General Characteristics and Localization of Antigens
The antigenic properties of blood cells depend on specific chemical structures on their surface. These structures are based on oligosaccharides that form part of the glycoprotein complexes of the outer cell membrane.
In immunohematology, several systems classify blood according to these features, but two are considered fundamental:
- AB0 system — of decisive importance in blood transfusions and the development of immune reactions.
- Rhesus system — divides all people into two categories: Rhesus-positive and Rhesus-negative.
It is important to note that blood group antigens are not exclusive to erythrocytes. They are also expressed on platelet membranes, cells of various body tissues, and are even secreted in fluids such as saliva, breast milk, and seminal fluid.
Characteristics of Blood Groups in the AB0 System
The AB0 system is based on the presence of two types of protein structures: antigens (aglutinogens) located on the erythrocyte membrane, and antibodies (agglutinins) circulating in the blood plasma. Depending on their combination, there are four main groups:
- Group I (0)
Only the basic 0 antigen is present on the erythrocyte surface. The blood plasma contains two types of antibodies simultaneously: anti-A and anti-B. This is the most common group, found in approximately 40% of people.
- Group II (A)
Erythrocytes carry the basic 0 antigen and the A antigen. Only anti-B antibodies are produced in the plasma. This phenotype is the most frequent, covering about 43% of the population.
- Group III (B)
Membranes of red blood cells contain 0 and B antigens. The plasma contains anti-A antibodies. This group is less common, carried by approximately 12% of people.
- Group IV (AB)
Blood cells are equipped with a full set of antigens: 0, A, and B. At the same time, group antibodies are completely absent from the plasma. This is the rarest group, characteristic of only 5% of the population.
Mechanisms of Group Incompatibility
Group incompatibility is the basis of severe complications during blood transfusions. The essence of the pathological reaction is agglutination — the clumping of erythrocytes into conglomerates, which is inevitably followed by their destruction (hemolysis).
This process is triggered when corresponding antigens and antibodies meet in the bloodstream (e.g., antigen A encounters anti-A). In clinical practice, two vectors of immune attack are distinguished:
- Recipient attack: the patient's own antibodies destroy donor erythrocytes. This is the most frequent and dangerous incompatibility mechanism.
- Donor attack: antibodies contained in the transfused donor plasma attack the patient's erythrocytes (occurring predominantly during whole blood transfusion).
- A clear example of incompatibility is the mixing of Group II (A) and Group III (B) blood. In this case, a massive cross-reaction is triggered: the recipient's anti-B antibodies bind to the donor's B-erythrocytes, and the donor's anti-A antibodies attack the recipient's A-erythrocytes.
Principles of Blood Transfusion
When transfusing small volumes of blood, the dilution rule applies: donor antibodies (agglutinins), upon entering the recipient's bloodstream, are diluted multiple times by the recipient's plasma, causing their concentration to drop below a critical level so they cannot damage the patient's cells.
Based on this, the main rule of safe transfusion is to prevent the interaction of recipient antibodies with the infused donor erythrocytes.
Basic compatibility rules for small volumes:
- Individuals with Group I (0) are considered universal donors because their erythrocytes lack A and B antigens, meaning they can be transfused to people with any blood group.
- People with Group IV (AB) are universal recipients because their plasma lacks antibodies capable of attacking foreign erythrocytes.
- Blood of Group II (A) and Group III (B) may be transfused to patients with the exact same blood group, or to recipients with Group IV (AB).