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Blood Types &
ABO/Rh Compatibility

Understand the ABO blood group system, the Rh factor, and why compatibility matters for transfusions and pregnancy.

Anatomy & Physiology

What Are Blood Types?

Your blood type is determined by specific antigens (proteins and sugars) on the surface of your red blood cells. These antigens act as molecular ID tags that your immune system uses to distinguish "self" from "foreign."

Two classification systems define your blood type: the ABO system (based on A and B antigens) and the Rh system (based on the Rh-D antigen). Together, they produce the 8 common blood types.

The ABO Blood Group System

Type A
Has A antigens on RBCs. Has anti-B antibodies in plasma.
Type B
Has B antigens on RBCs. Has anti-A antibodies in plasma.
Type AB
Has both A and B antigens. Has no ABO antibodies in plasma.
Type O
Has no A or B antigens. Has both anti-A and anti-B antibodies.

The Rh Factor: Positive vs. Negative

The Rh system adds a second layer of classification. The Rh-D antigen is either present (Rh+) or absent (Rh-) on your red blood cells. This doubles the ABO types from 4 to 8.

Rh+ (Positive) Rh-D antigen present Rh- (Negative) Rh-D antigen absent vs

The 8 Common Blood Types

ANTIGEN ON RBC → ANTIBODY IN PLASMA A+ A, Rh-D | anti-B A- A | anti-B B+ B, Rh-D | anti-A B- B | anti-A AB+ A, B, Rh-D | none AB- A, B | none O+ Rh-D | anti-A, anti-B O- none | anti-A, anti-B APPROXIMATE U.S. POPULATION DISTRIBUTION A: ~40% (A+ 34%, A- 6%) B: ~11% (B+ 9%, B- 2%) AB: ~4% (AB+ 3%, AB- 1%) O: ~45% (O+ 38%, O- 7%) Dashed border = Rh-negative subtype

The Golden Rule of Transfusion

1
Check the recipient's antibodies. What will their plasma attack?
2
Check the donor's antigens. What is on the surface of the donated RBCs?
3
If recipient antibodies match donor antigens: incompatible. The antibodies will attack and destroy the donor cells.

What Happens During Agglutination?

COMPATIBLE Donor RBC (no A/B antigen) Y anti-A no bind ✓ Cells flow freely INCOMPATIBLE Donor RBC (has A antigen) Y anti-A BINDS! Agglutination (clumping)

Universal Donor vs. Universal Recipient

O- : Universal Donor
O- red blood cells have no A, B, or Rh-D antigens. No recipient antibody can attack them. Safe for all 8 blood types in emergencies.
AB+ : Universal Recipient
AB+ plasma has no anti-A, anti-B, or anti-Rh antibodies. It will not attack any donated RBCs. Can receive from all 8 blood types.

ABO/Rh Compatibility Chart

WHO CAN DONATE TO WHOM? (RBC Transfusion) BLOOD TYPES O- Donates to ALL O+ → Rh+ types A+ A- B+ B- AB+ Receives from ALL AB- Lines show donation pathways (donor → recipient). Dashed = universal paths.

Antigens vs. Antibodies: The Key Distinction

Antigens Antibodies
Location Surface of red blood cells Dissolved in plasma
Chemical nature Glycoproteins / glycolipids Immunoglobulins (IgM, IgG)
Function "ID tag" on the cell "Security guard" that attacks foreign IDs
Determines What blood type you are What blood type you reject
Transfusion focus Donor's antigens matter Recipient's antibodies matter

How to Determine Compatibility

1
Identify the recipient's blood type. What antigens do they have? What antibodies are in their plasma?
2
Identify the donor's blood type. What antigens are on the donated RBCs?
3
Check for conflict: Do any of the recipient's antibodies match the donor's antigens? If anti-A meets A antigen, or anti-B meets B antigen: incompatible.
4
Check Rh: Can an Rh- recipient receive Rh+ blood? No (risk of sensitization). Can an Rh+ recipient receive Rh- blood? Yes.
5
No conflicts? The transfusion is compatible.

Example 1: Simple Compatibility

Scenario: A patient with blood type B+ needs a transfusion. Can they receive O+ blood?

1
Recipient: B+ has B antigens and Rh-D on RBCs. Plasma contains anti-A antibodies.
2
Donor: O+ has no A or B antigens, but has Rh-D on RBCs.
3
ABO check: Recipient's anti-A looks for A antigen on donor cells. O+ has no A antigen. ✓ No conflict.
4
Rh check: Recipient is Rh+. Donor is Rh+. Rh+ to Rh+ is always safe. ✓ No conflict.

Example 2: Incompatible Transfusion

Scenario: A patient with blood type O- needs blood. Can they receive A+ blood?

1
Recipient: O- has no antigens on RBCs. Plasma contains anti-A and anti-B antibodies. No Rh antibodies yet, but is Rh-.
2
Donor: A+ has A antigen and Rh-D on RBCs.
3
ABO check: Recipient's anti-A will bind the donor's A antigen. ✕ CONFLICT!
4
Rh check: Rh- recipient receiving Rh+ blood risks sensitization. ✕ CONFLICT!

Example 3: Rh Factor in Pregnancy

Scenario: An Rh- mother carries an Rh+ fetus. What risk does this create?

Mother (Rh-) No Rh-D antigen on her RBCs May develop anti-Rh Fetus (Rh+) Has Rh-D antigen on its RBCs Target of attack Fetal Rh+ cells leak 2nd Pregnancy: Mother's anti-Rh attacks fetal RBCs Hemolytic Disease of the Newborn (HDN)

Common Mistakes to Avoid

✕ Confusing antigens with antibodies
Antigens are on cells. Antibodies are in plasma. Mixing them up reverses every compatibility answer.
✕ Thinking O+ is the universal donor
O- is universal. O+ has Rh-D antigen, so it cannot go to Rh- recipients without risk of sensitization.
✕ Forgetting Rh in compatibility
Students check ABO but skip Rh. An Rh- patient receiving Rh+ blood is a mismatch even if ABO is fine.

Summary Compatibility Table

Blood Type Can Donate To Can Receive From
O- All 8 types O- only
O+ O+, A+, B+, AB+ O-, O+
A- A-, A+, AB-, AB+ O-, A-
A+ A+, AB+ O-, O+, A-, A+
B- B-, B+, AB-, AB+ O-, B-
B+ B+, AB+ O-, O+, B-, B+
AB- AB-, AB+ O-, A-, B-, AB-
AB+ AB+ only All 8 types

Question 1 of 5

A person with blood type AB+ has which of the following in their plasma?

Question 2 of 5

Which blood type is considered the universal red blood cell donor?

Question 3 of 5

A patient with blood type B- receives a transfusion of A- blood. What is the most likely immediate result?

Question 4 of 5

An Rh-negative mother pregnant with an Rh-positive fetus is at risk of developing antibodies that could harm the baby. What is this condition called, and how is it prevented?

Question 5 of 5

A trauma patient arrives with unknown blood type. The ER has only O+ and O- packed red blood cells available. Which should be given, and why?