The universal blood donors for the abo system are type

     Blood-type in the ABO system is determined by three alleles  (IA , IB , IO) at the Isoagglutinin locus on Chromosome 9. The IA and IB alleles determine antigenic proteins on the surface of red blood cells, which in turn cause the immune system to develop antibodies against the antigen not present. An individual who isIAIB has both antigens, and therefore does not generate antibodies to either: alleles IA and IB are thus co-dominant. Individuals who areIAIA orIAIO have the same blood type and generate anti-B antibodies: IA is thus dominant to IO.  The same principles apply toIBIB and IBIO, such that IB is dominant to IO. An individual who isIOIO has no antigens, and generates antibodies to both.

    The test for ABO blood type involves mixing purified A' (anti-A) and B' (anti-B) antibodies with the blood to be typed. Type A blood coagulates in the presence of A' antibody only, type B reacts with B' antigen only, type AB with neither, and type O with both. The same reaction would occur in vivo, which necessitates typing of blood prior to transfusion. Persons with an IA or IB allele cannot receive blood from a person with either an IB or an IA allele, respectively. That is, type A persons cannot receive Type B or AB blood, and vice versa.

    In TV medical shows, the frequent request for "O negative" blood reflects the fact that Type O blood is compatible with any ABO blood type: Type O is thus described as the 'universal donor'. Type AB is a 'universal recipient' due to the lack of antibody production, which is fortunate because the AB type is the rarest.

    "Negative" refers to the second major blood-type locus, the Rh (Rhesus ) factor, with two alleles Rh+ and Rh-, where the first is a simple dominant to the second and produces an antigen absent in the second. Following the same principles as in ABO,  Rh- blood can be safely transfused to any recipient. Note that Rh- persons constitute only  ~15% of the North American population.

How do blood types work?

Your blood group is inherited from genes passed on by your parents.

The two most important blood group systems for transfusion are ABO and Rh.

The ABO group system gives the letter part of your blood type and Rh system gives the positive or negative part.

The ABO group system

There are four main blood groups in the ABO group system.

Each group is important and we need donors from every group to ensure we have the right blood for the people who need it.

Which group you belong to depends on the antigens and antibodies in your blood.

Antigens are a combination of sugars and proteins that coat the surface of a red blood cell.

Depending on your combination, you will have:

• no antigens
• A antigens
• B antigens
• both A and B antigens

You also have antibodies in the clear part of blood called plasma.

Antibodies are important in transfusion because they are the body’s natural defence system. They recognise any ‘foreign’ antigens and tell your immune system to destroy them.

That’s why giving someone blood from the wrong ABO group can be life-threatening.

It is the presence of A and B antigens and corresponding antibodies that determines your ABO blood group.

Is O+ or O universal donor?

Why O+ is called universal donor?