Explain ABO and Rh typing, antibody screens and crossmatches, and choose compatible red cells and plasma.
Why it matters
The blood bank (immunohematology) makes sure that a patient who receives a transfusion gets blood their immune system will not attack. ABO-incompatible transfusion can cause an acute hemolytic reaction that can kill, and most such events trace back to identification errors, not test failures. That is why blood bank work is the most procedure-driven area of the lab.
Safety and scope. Blood bank testing is done only in an accredited laboratory, by trained and competency-assessed staff, under the supervision of a medical director. This lesson is knowledge only and is not medical advice.
ABO and Rh
Red cells carry antigens on their surface. In the ABO system:
| Blood group | Antigens on red cells | Antibodies in plasma |
|---|---|---|
| A | A | anti-B |
| B | B | anti-A |
| AB | A and B | none |
| O | none | anti-A and anti-B |
People make ABO antibodies naturally in early life, which is why ABO mismatch is so dangerous. The Rh(D) antigen is either present (Rh-positive) or absent (Rh-negative). Rh-negative people usually make anti-D only after exposure, for example through transfusion or pregnancy.
Typing: forward and reverse must agree
- Forward type: the patient's red cells are mixed with anti-A and anti-B reagents. Agglutination (clumping) shows which antigens are present.
- Reverse type: the patient's plasma is mixed with known A1 and B reagent red cells. Agglutination shows which antibodies are present.
The two must agree. A group A patient shows agglutination with anti-A (forward) and with B cells (reverse). If they disagree (an ABO discrepancy), the lab must resolve it before issuing type-specific blood; until then, the patient receives group O red cells if transfusion cannot wait. Discrepancies have causes such as weak antibodies in newborns and older adults, recent transfusion, or a mislabelled sample. Many hospitals also require a second, separately drawn sample to confirm the type before issuing non-O red cells.
Antibody screen and crossmatch
- Antibody screen: the patient's plasma is tested against two or three reagent red cells that carry the clinically important non-ABO antigens (such as Kell, Duffy and Kidd). A positive screen means the lab must identify the antibody and find antigen-negative units, which can take hours.
- Crossmatch: a final compatibility check between the patient and a specific donor unit.
- Immediate-spin crossmatch: patient plasma plus donor cells, spun and read at once; it mainly confirms ABO compatibility.
- Antiglobulin (AHG) crossmatch: includes incubation and anti-human globulin to detect IgG antibodies; used when the patient has a clinically significant antibody.
- Electronic crossmatch: a validated computer check of ABO compatibility, allowed only when the patient has a negative screen and two concordant types on record.
A type and screen is often ordered before surgery; a crossmatch follows only if blood is likely to be needed.
Worked example: choosing compatible units
A fictional patient types as B, Rh-negative with a negative antibody screen.
- Compatible red cells: group B or O (no A antigen, because the patient has anti-A), and Rh-negative.
- Compatible plasma: group B or AB (plasma must not contain anti-B, because the patient's cells carry B).
The pattern reverses between red cells and plasma: group O red cells suit everyone (no A or B antigens), while group AB plasma suits everyone (no anti-A or anti-B). In an emergency before the type is known, labs issue O red cells (often O-negative for females of childbearing potential) and AB plasma, per policy.
Common mistakes
- Issuing type-specific blood while a forward/reverse discrepancy is unresolved.
- Confusing red-cell and plasma compatibility rules.
- Accepting a blood bank sample with any labelling defect. Blood bank labelling rules are the strictest in the lab, and a defect means a new sample.
- Using an electronic crossmatch for a patient with a positive antibody screen.
Quick recap
- ABO antibodies are naturally present; forward and reverse types must agree.
- Antibody screens detect clinically significant non-ABO antibodies.
- Crossmatch: immediate spin, AHG, or electronic (only when eligible).
- Red cells: O is universal. Plasma: AB is universal. Identification errors are the main danger.
Try it
Open the Do tab for the quiz: match patients to compatible units.