3.3 Blood Types and Transfusion

ABO and Rh Blood Groups

ABO Blood Group System

The ABO system classifies blood based on which antigens sit on the surface of your red blood cells. These antigens are glycoproteins, and your immune system uses them to distinguish "self" from "foreign." If foreign antigens show up in your blood (say, from a transfusion), your antibodies will attack them.

There are four main ABO blood types:

• Type A — has A antigens on RBCs; produces anti-B antibodies in plasma
• Type B — has B antigens on RBCs; produces anti-A antibodies in plasma
• Type AB — has both A and B antigens; produces no ABO antibodies (universal plasma recipient)
• Type O — has neither A nor B antigens; produces both anti-A and anti-B antibodies

Notice the pattern: you naturally produce antibodies against whichever ABO antigen you don't have. These antibodies form early in life without prior exposure to foreign blood, which is why ABO mismatches are so dangerous even on a first transfusion.

Genetics of ABO type: The ABO gene has three alleles:

• $I^A$ codes for the A antigen
• $I^B$ codes for the B antigen
• $i$ codes for no antigen

$I^A$ and $I^B$ are codominant (both expressed when inherited together, producing type AB). The $i$ allele is recessive, so you need two copies ($ii$) to be type O.

Rh Blood Group System

The Rh system is based on the presence or absence of the D antigen (commonly called the "Rh factor") on red blood cells.

• Rh-positive (Rh+): D antigen is present
• Rh-negative (Rh−): D antigen is absent

The RHD gene is dominant, so only one copy of the RHD allele is needed for an Rh-positive phenotype. You must inherit two recessive alleles to be Rh-negative.

Unlike the ABO system, Rh-negative individuals do not naturally carry anti-D antibodies. They only develop them after exposure to Rh-positive blood (through transfusion or pregnancy). This distinction matters clinically.

Since ABO and Rh are inherited independently, combining them gives eight possible blood types: A+, A−, B+, B−, AB+, AB−, O+, and O−.

Blood Typing in Transfusion

Importance of Blood Typing

Blood typing determines a person's ABO and Rh group before any transfusion. Getting this wrong can be fatal.

ABO incompatibility triggers an immediate immune response. For example, if type A blood is transfused into a type B recipient, the recipient's pre-existing anti-A antibodies bind to the donated red blood cells, causing massive agglutination (clumping) and hemolysis (cell destruction). Because ABO antibodies are already circulating, this reaction happens fast.

Rh incompatibility in pregnancy is a separate but critical concern. When an Rh-negative mother carries an Rh-positive fetus, fetal red blood cells can enter the mother's circulation (especially during delivery). The mother's immune system then produces anti-D antibodies. In a subsequent Rh-positive pregnancy, those antibodies can cross the placenta and destroy fetal red blood cells, causing hemolytic disease of the newborn (HDN). This is prevented by administering RhoGAM (Rh immunoglobulin) to Rh-negative mothers during and after pregnancy.

Cross-Matching Procedure

Even after blood typing, cross-matching is performed to catch incompatibilities that typing alone might miss (such as antibodies to minor blood group antigens).

1. Major cross-match: The recipient's serum is mixed with the donor's red blood cells. If agglutination or hemolysis occurs, the recipient has antibodies that would attack the donor's cells. This is the most critical test.
2. Minor cross-match: The donor's serum is mixed with the recipient's red blood cells. This checks whether the donor's plasma contains antibodies that could harm the recipient's cells.
3. If neither test shows a reaction, the blood is considered compatible for transfusion.

Blood Transfusion Complications

Acute Hemolytic Transfusion Reactions

These occur when the recipient's antibodies destroy the transfused red blood cells. ABO incompatibility is the most common cause.

The antibody-antigen reaction triggers complement activation, leading to intravascular hemolysis. Symptoms include fever, chills, flank/back pain, and dark or red-brown urine (from free hemoglobin). In severe cases, the cascade can lead to disseminated intravascular coagulation (DIC), renal failure, shock, and death.

If a hemolytic reaction is suspected, the transfusion must be stopped immediately.

Non-Hemolytic Transfusion Reactions

Not all transfusion reactions involve red blood cell destruction:

• Febrile non-hemolytic reactions cause fever and chills during or shortly after transfusion. They're typically caused by the recipient's antibodies reacting to donor white blood cells or cytokines that accumulated during storage. These are uncomfortable but not usually dangerous.
• Allergic reactions range from mild urticaria (hives) to severe anaphylaxis. They result from the recipient's immune response to proteins in the donor's plasma. Mild cases respond to antihistamines; anaphylaxis requires epinephrine and immediate intervention.
• Transfusion-related acute lung injury (TRALI) is a serious complication causing acute respiratory distress and pulmonary edema within 6 hours of transfusion. It's thought to be triggered by donor antibodies reacting with the recipient's white blood cells, causing inflammation and fluid buildup in the lungs. TRALI is a leading cause of transfusion-related death.

Transfusion-Transmitted Infections

Donated blood is screened for pathogens including HIV, hepatitis B, hepatitis C, and syphilis. Modern testing has made the blood supply very safe, but a small residual risk remains due to the window period, the time between a donor's initial infection and when the pathogen becomes detectable by lab tests.

Nucleic acid testing (NAT) has significantly shortened window periods compared to older antibody-based screening, but no test eliminates risk entirely.

Blood Donation and Transfusion Procedures

Blood Donation Process

Blood donation collects either whole blood or specific components from a voluntary donor. Before collection, donors go through a screening process:

• A health questionnaire covering medical history, travel, medications, and high-risk behaviors
• A mini-physical (blood pressure, pulse, temperature, hemoglobin check)
• Laboratory testing of the donated unit for infectious diseases

After collection, whole blood is typically separated into components, each with different storage requirements:

Specialized Donation Procedures

• Autologous donation: You donate your own blood before a planned surgery so it can be transfused back to you during or after the procedure. This eliminates the risk of ABO/Rh incompatibility and transfusion-transmitted infections.
• Apheresis: A machine draws your blood, separates out a specific component (platelets, plasma, or red blood cells), and returns the rest to you. This allows donors to give larger quantities of a single component and to donate more frequently than with whole blood donation.

Transfusion Administration

The blood product chosen depends on what the patient needs:

• Packed red blood cells for anemia or acute blood loss
• Platelets for thrombocytopenia (low platelet count) or platelet dysfunction
• Fresh frozen plasma for coagulation factor deficiencies or as volume replacement in massive transfusions

Safe transfusion administration follows a strict protocol:

1. Verify the patient's identity using at least two identifiers (name, date of birth, medical record number).
2. Confirm the blood product label matches the patient's blood type and the compatibility report.
3. Take baseline vital signs before starting the infusion.
4. Begin the transfusion slowly for the first 15 minutes while monitoring closely for early signs of a reaction (fever, chills, itching, shortness of breath).
5. Continue monitoring vital signs at regular intervals throughout and after the transfusion.

If any signs of an adverse reaction appear, stop the transfusion immediately, maintain IV access with normal saline, and notify the provider.