ABO Blood Type System Explained: A, B, AB, and O Types
The ABO Antigens
ABO blood type is defined by the presence (or absence) of A and B antigens on red blood cell surfaces. Type A: has A antigen. Type B: has B antigen. Type AB: has both. Type O: has neither. These antigens are complex carbohydrate structures attached to glycoproteins on the red cell surface.
The Antibody Rule: The Opposite of What You Have
Your immune system naturally produces antibodies against whichever ABO antigens you lack. Type A: has anti-B antibodies. Type B: has anti-A antibodies. Type AB: has neither anti-A nor anti-B. Type O: has both anti-A and anti-B. This is why transfusing incompatible blood causes immediate immune reaction — the pre-existing antibodies attack the transfused cells.
Genetics: Inheriting Blood Type
Blood type is controlled by the ABO gene with three alleles: I^A, I^B, and i. I^A and I^B are codominant; i is recessive. Type A: I^A I^A or I^A i. Type B: I^B I^B or I^B i. Type AB: I^A I^B (both expressed). Type O: ii. This means two Type A parents can have a Type O child (if both are I^A i).
Blood Type Distribution Globally
O+ is the most common worldwide (38–40%). A+ second (27–32%). B+ third (8–25%, more common in Asian populations). AB+ rarest ABO (3–5%). Rh negative types are rare in African and Asian populations (2–4%) but more common in European populations (15%). Use Blood Type Compatibility to see your type’s specific donor and recipient matches.
Frequently Asked Questions
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