It is seldom that the blood type itself is rare; it is the antigens in the blood that more often make the blood rare. So it’s like having a rare recipe to your blood.
There are more than 600 known antigens besides A and B that identify the proteins found on a person's red blood cells. If an individual's blood has uncommon antigens or lacks common antigens, the person should be tested and categorized as having a rare blood type. So it’s what’s in your blood in addition to the ABO types and the Rhesus positive or negative part which decides whether you are rare or not.
A rare blood type is any blood type that is difficult to find in the population where you may need that "rare" type of Blood. One way of defining a blood type as rare is when more than 200 donors must be screened to find one compatible donor with blood of that desired type. So it’s a needle in a haystack.
To be more precise, an individual's blood type is most often considered to be rare if only one other person in 1,000 lacks the same antigens or shares the same uncommon antigens. A person's blood type is considered as very rare if only one person in 10,000 has or lacks similar blood antigens. That’s a much bigger haystack to search in!
There are racial and ethnic differences in blood type and composition. Your blood type is inherited in the same way as your eye and hair colour. That is why it is almost impossible to find a rare blood type that is needed to transfuse an Asian patient, for example, in a donor who is white, and vice versa.
For example there are certain ethnic groups where certain blood types are most common. This is to do with geography and ethnicity combining. Examples are:
African / Caribbean / black community in Amercia U- and Duffy-
Native Americans and Alaskan Native peoples RzRz
Pacific Island peoples and Asian Jk ( a- b- )
Hispanics Di ( b- )
Russian Jews Dr ( a- )
Caucasians Kp ( b- ) and Vel
So where you are in the world can have a brearing on what is considered a rare type and what blood is available. In the UK we especially need more black and Asian donors, because we have more black and Asian patients who require this blood than donors.
When a patient requires a blood transfusion, they can usually be given blood from a donor based on their ABO group and Rh type only.
With some medical conditions, however, patients can require ongoing multiple blood transfusions, sometimes for the rest of their lives. These patients need to be given more extensively matched blood. So, if a patient has the Ro subtype and requires ongoing multiple transfusions, usually they need blood of the Ro subtype. When this is not possible they receive blood which is less well matched, a bit like asking for a skinny decaf machiatto but instead receiving a flat white decaf. You’ve still got your coffee and it does the same job but it’s not the exact an order as you requested. For patients receiving less well matched blood over a long period this can cause complications in their condition, so its im[portant we get the best possible matches.
Not everybody knows their blood type and nearly nobody knows their subtypes but what we do know is that you’re most likely to share rare blood types with donors from the same ethnic heritage. So for Sickle Cell disease patients who are mainly from black heritage and for Thalassaemia patients who are mainly from Asian, Arab or Mediterranean heritage, we know we need more donors from these same heritages.