Blood types are determined by the presence or absence of certain antigens on the surface of red blood cells. The two most important blood group systems are the ABO system and the Rh system. The ABO system has four blood types: A, B, AB, and O. The Rh system has a positive (Rh+) and negative (Rh-) type. When we talk about blood types, the ABO type is stated first, followed by the Rh type. So someone who is AB positive would have type AB+, while someone who is O negative would have type O-.
ABO Blood Group System
The ABO blood group system is determined by the presence or absence of the A and B antigens on the surface of red blood cells:
- Group A has only the A antigen on red cells (and B antibody in the plasma)
- Group B has only the B antigen on red cells (and A antibody in the plasma)
- Group AB has both A and B antigens on red cells (but neither A nor B antibody in the plasma)
- Group O has neither A nor B antigens on red cells (but both A and B antibody in the plasma)
The ABO system is inherited – a person’s blood type is determined by the blood types of their parents. The A and B alleles are co-dominant, so both can be expressed if they are present. The O allele is recessive to A and B.
ABO Blood Type Frequencies
The frequencies of the major ABO blood types vary around the world, with type O being the most common globally. Here are the approximate frequencies:
- O+: 37%
- O-: 7%
- A+: 36%
- A-: 6%
- B+: 9%
- B-: 2%
- AB+: 3%
- AB-: 1%
So together, O+ and O- make up about 44% of the population.
Rh Blood Group System
The Rh system refers to the presence or absence of the D antigen on red blood cells. If your blood has the D antigen, you are Rh positive. If your blood lacks the D antigen, you are Rh negative.
Being Rh positive or negative is also inherited from your parents. The D allele is dominant – if you inherit a D allele from either parent, you will be Rh positive. You need to inherit two recessive d alleles (one from each parent) to be Rh negative.
Rh positive is the most common type, with about 85% of people being Rh positive and 15% being Rh negative.
Importance of the Rh Factor
The most significant complication of the Rh system occurs during pregnancy. If a woman is Rh negative and carrying a Rh positive fetus, her immune system may produce antibodies against the Rh antigen. This is not an issue in the first pregnancy, but can lead to hemolytic disease of the newborn in subsequent pregnancies.
For this reason, Rh negative pregnant women receive a medication called RhoGAM to prevent sensitization. RhoGAM works by binding any fetal Rh positive cells that enter the mother’s bloodstream, preventing her from producing antibodies against the Rh antigen.
Comparing O+ and O- Blood Types
Now that we’ve reviewed the ABO and Rh blood group systems, let’s directly compare O+ and O- blood types:
Similarity – Lack of A and B Antigens
The most significant similarity between O+ and O- blood is the lack of A and B antigens on the red blood cells. This means that both O+ and O- blood can be safely transfused to people with any ABO blood type. People with O- are considered “universal donors” for red blood cell transfusions.
Difference – Presence or Absence of Rh D Antigen
The key difference between O+ and O- blood comes down to the Rh factor. O+ blood is Rh positive, meaning it contains the D antigen. O- blood is Rh negative and lacks the D antigen.
This means O- can be safely transfused to people who are Rh negative, while O+ should only be given to Rh positive recipients. O- is considered the “universal donor” for whole blood transfusions because it contains neither A/B antigens nor the Rh D antigen.
| Blood Type | A Antigen | B Antigen | Rh D Antigen |
|---|---|---|---|
| O+ | Absent | Absent | Present |
| O- | Absent | Absent | Absent |
Transfusion Compatibility
Here is a table summarizing the transfusion compatibility for O+ and O- blood types:
| Donor Blood Type | Compatible Recipient Blood Types |
|---|---|
| O+ | O+, A+, B+, AB+ |
| O- | O-, O+, A+, A-, B+, B-, AB+, AB- |
As shown, O- is compatible with all blood types, while O+ can only be given to Rh positive recipients.
Frequency
O+ is the most common blood type, found in about 37% of the population. O- is much rarer at around 7% of people. So while they share the lack of A and B antigens, O- blood is the less common subtype.
Advantages of O- Blood
While neither blood type is necessarily “better” overall, O- blood does have some advantages:
- O- can be safely transfused to anyone in emergency situations when blood type is unknown
- O- donors are universal donors for blood and plasma
- There is typically a shortage of O- supply at blood banks since only 7% of people have it
- O- is routinely given to preterm infants and patients with weakened immune systems who require multiple transfusions
- Women who are O- can receive O- blood safely during pregnancy, without the risk of Rh sensitization
For these reasons, people with O- blood are highly encouraged to donate blood to help increase the O- supply.
Disadvantages of O- Blood
However, there are some disadvantages to having O- blood:
- While O- individuals are universal donors, they can only receive O- blood
- They have a higher risk of hemorrhage if sufficient O- blood is not available
- O- patients may have longer waits to receive properly matched blood
- Women who are O- need to take extra precaution to avoid Rh sensitization during pregnancy
So people with O- blood need to take care to ensure they have access to O- supply. Some facilities encourage O- patients to store their own blood prior to surgeries in case transfusions are needed.
Advantages of O+ Blood
O+ blood also has some advantages:
- It is the most common blood type, so O+ supply is readily available
- O+ can be transfused to Rh positive patients with any ABO blood type
- Women who are O+ do not risk Rh sensitization during pregnancy
- People with O+ blood have a wider selection of blood to receive compared to O- individuals
The high frequency and compatibility with all Rh positive blood types provides advantages in the ease of finding compatible O+ blood.
Disadvantages of O+ Blood
Some disadvantages of O+ blood include:
- While O+ people can donate to all Rh positives, they can only receive O+ blood
- They cannot donate to Rh negative patients
- Hospitals typically have sufficient O+ supply, so donations are often needed more for rarer types like O-
So people with O+ blood do not confer the same universal donor lifesaving benefits as those with O-. Their donations, while still needed, are less crucial for maintaining rare supply.
Conclusion
In summary, neither O+ or O- blood type is necessarily “better” overall. O- has the advantage of being a universal donor, but the disadvantage of having a rare supply. O+ is much more common, but can only be given to Rh positive recipients.
For blood donation and transfusion matching purposes, O- is the most precious blood type while O+ is readily available. But from an individual health perspective, neither O+ nor O- confer any special advantages or disadvantages outside of transfusion medicine. Both are common, normal, and healthy blood types.