Blood Type Compatibility Calculator

Blood type compatibility is set by the ABO blood group system and the Rh factor, two independent genetic traits that together produce 8 common blood types. Compatibility matters for blood transfusions, organ transplants, and pregnancy planning.

ABO System formula:

Blood type is determined by antigens present on red blood cell surfaces and antibodies in plasma:

• Type A: has A antigens, anti-B antibodies
• Type B: has B antigens, anti-A antibodies
• Type AB: has both A and B antigens, no antibodies (AB positive is the universal recipient for red cells)
• Type O: has no ABO antigens, both anti-A and anti-B antibodies (O negative is the universal donor for red cells)

Rh Factor:

• Rh+ (positive): Rh antigen present on red cells (more common)
• Rh− (negative): Rh antigen absent

Blood transfusion compatibility (red blood cells):

Plasma compatibility is reversed: AB plasma can donate to all types; O plasma can only go to type O recipients. The Rh factor drops out for plasma, because it sits on the red cells and plasma has none.

Emergencies before the type is known. Group O red cells go first. O negative is the safe choice for anyone, but only about 7% of people have it, so many trauma centres give O positive to men and older women and keep O negative for girls and women who could still become pregnant. That is the group where an Rh mismatch could cause trouble in a later pregnancy.

Inheritance formula:

Blood type is inherited from both parents. Each parent contributes one allele (A, B, or O). A and B are dominant over O; A and B are codominant with each other.

• Two O parents → child is always O
• One A parent (AO) + one B parent (BO) → child can be A, B, AB, or O

Worked example: inheritance

Mother is type A (AO genotype). Father is type B (BO genotype).
Possible child genotypes: AO (type A), BO (type B), AB (type AB), OO (type O).
Each outcome has a 25% probability, so all four blood types are possible from this one couple.

Rh disease in pregnancy: An Rh− mother carrying an Rh+ baby can develop anti-Rh antibodies. Subsequent pregnancies may be at risk. Rh immunoglobulin (RhoGAM) is given to prevent sensitization, usually at about 28 weeks, again within 72 hours of the birth if the baby is Rh positive, and after any bleeding, miscarriage or injury in pregnancy.