Blood Types. The ABO and Rhesus Systems

January 24, 2023

Are you an A+ or a B-? These are not your school grades — but your blood types. Blood types are the antigens (sugars or proteins) found on the surface of red blood cells (RBCs). An antigen is a substance that induces an immune response. The type of antigen on the RBCs determines your blood type.

Another term related to but different from the antigen is an antibody. Antibodies to blood group antigens are found in your plasma. As part of your immune system, antibodies tend to identify antigens on the surfaces of RBCs. If the antigen identified is foreign, antibodies launch an attack to defend you from that antigen. This attack causes your RBCs to break — a process termed hemolysis — that induces destruction.

There are about 36 recognized blood systems today. This article will cover the two central blood type systems: The ABO and Rhesus systems.

What is the ABO blood group system?

Now that we know what an antigen and antibody are, let's talk about which antigens and antibodies make up the ABO system. This system consists of:

Two antigens named antigen A and antigen B on RBCs. These antigens:

May be present alone or as a combo or be absent.
Are sugars.

Two antibodies called anti-A and anti-B antibodies in the plasma. These antibodies:

Occur naturally in your body when there are no corresponding antigens (see below).
Will trigger RBC breakdown when they encounter ABO-incompatible blood cells.

What is the Rhesus (Rh) blood group system?

This system comprises:

Rh antigens, of which the most clinically significant is the Rh(D) antigen or factor.

Rh factor is a protein found on the RBCs and indicated by the positive or negative sign next to the letters in your blood group.
You’ll be labeled as Rh+ if your blood contains the Rh factor and Rh- if there’s no Rh factor.
Antigens of the rhesus system are proteins.

Anti-Rh antibodies

Form only in rhesus-negative people after exposure to rhesus-positive antigen.
Will trigger RBC breakdown when they encounter rhesus incompatible blood cells.

What are the different blood types?

Depending on the genes inherited from your parents, you may fall under one of the four primary blood types:

Type A
Type B
Type AB
Type O

Each of the four ABO blood types can then be either Rh+ or Rh-, depending on the presence or absence of the Rh factor.

This leaves us with eight blood types as follows:

A+
A-
B+
B-
O+
O-
Type AB+
Type AB-

Why are blood types important?

Before delving into the features of different blood types, let’s first discuss the two most common scenarios where knowing your blood type is important.

1. To prevent mismatching during blood transfusion

Knowing your blood type is especially helpful in emergencies such as emergency blood transfusion or donation.

If you receive a blood type that is incompatible with (doesn’t match) your blood type (which rarely happens now), your body will recognize the donor’s blood cells as foreign and produce antibodies to destroy them. This process triggers the fatal transfusion reaction. Accurately matching your blood with the donor’s blood is therefore imperative before a transfusion.

2. To prevent Rhesus (Rh) incompatibility during pregnancy

Another critical scenario in which knowing the blood type is critically helpful is during pregnancy. It is essential to know the mother’s and baby’s blood types because problems arise if the mother is Rh-negative, and the baby is Rh-positive. There are several stages in pregnancy when the Rh-positive fetal red cells can enter the mother’s system, such as during labor.

The mother’s immune system will treat the Rh-positive fetal red cells as foreign and make anti-D antibodies to damage the baby's red blood cells. Medical experts refer to this process as Rh incompatibility.

Hence, it’s a routine practice administering anti-D antibody injection to Rh-negative pregnant women that blocks the antibodies from harming the baby’s red blood cells.

Overview of the blood types

Here is a rundown of important features of the eight blood types:

Type A+

It is one of the most common blood types, found in about 35.7% of the U.S. population.
Has the antigen A and Rh factor on the RBCs (and anti-B antibodies in the plasma).
Can donate blood to A+ and AB+.
Cannot donate blood to any of the B blood types because anti-B antibodies will attack the B antigen in the B blood types.
Cannot donate blood to the O blood types. O blood types cannot recognize the A antigen in the A blood type and will launch an attack because they don’t have this antigen (see below).
Can receive blood from all A and O blood types (i.e., from A+, A-, O+, and O-).

Type A-

Much rarer than the A+ blood type, seen in only 6% of the US population.
Has the antigen A but no Rh antigen on the RBCs (and harbors anti-B antibodies in the plasma).
Can donate blood to all A and AB types, positive or negative.
Cannot donate blood to any of the B blood types because anti-B antibodies in the A blood types will attack the B antigen.
Cannot also donate blood to the O blood types because the antibodies in O blood types will identify the A antigen of the A blood types as foreign, launching an attack (see below).
May receive transfusion from only blood types A- and O- but not the B blood types because anti-B antibodies in A blood type will recognize the B blood type's antigen as foreign and launch an attack.

Type B+

This is also one of the rarer types, with only around 9% of the population carrying B+ blood.
Has the antigen B and Rh factor on RBCs (and anti-A antibodies in the plasma).
Can donate blood to both B+ and AB+ blood types.
Can receive blood from B+, B-, O+, and O- donors.
Cannot donate or receive blood to or from A blood types because anti-A antibodies in B blood types will recognize the A antigen of the A blood type as foreign and launch an attack. Cannot also donate blood to the O blood types. The anti-B antibodies in O blood types will identify the B antigen of the B blood types as foreign, launching an attack (see below).

Type B-

It is one of the rarest blood types — only 1.5% of the US population has it.
Has the antigen B but no Rh factor on the RBCs (and anti-A antibodies in the plasma).
Can donate blood to all B and AB types and receive types O- and B-.

Type O+

It is one of the most common blood types — 37.4% of the US population is type O+.
Has no A or B antigens but contains the Rh antigen on RBCs (and harbors both anti-A and anti-B antibodies in the plasma).
It is universally compatible with and can provide blood to all the Rh+ ABO types (A+, B+, O+, and AB+) since it doesn’t carry any A or B antigens to react with these blood types.
Not compatible with Rh- blood types because the rhesus factor in the O+ blood will attack the Rh- blood types.
It can only receive O blood group. If it receives A or B blood types, the anti-A and anti-B antibodies in the O blood type will attack the A and B antigens of the corresponding blood types.

Type O-

It is one of the rarest blood types — found in only 6.6% of the US population.
Neither has any A and B antigens nor any Rh factor on the RBC surface that could otherwise react with ABO and anti-rhesus D (RhD) antibodies. Hence, it is safe to donate to all blood types.
Because it is the universal donor, meaning anyone can receive type O- blood, it is often in high demand and short supply for emergencies where the blood type is unknown and time or resources are limited, so it’s often in short supply.
It harbors both anti-A and anti-B antibodies; thus, it can receive only O-negative type blood (anti-A and anti-B antibodies will react with the A and B antigens of all the A and B blood types, respectively).

Type AB+

It is one of the rarest blood types — accounting for only 3.4% of the US population.
Has both A and B antigens and the Rh factor on the RBCs (but no antibodies in the plasma that will recognize type A or B surface molecules).
It is a universal red cell recipient, meaning it can receive blood from all the blood types because it has no A and B antibodies to attack the corresponding antigens.
It is also a universal plasma donor, meaning it can give plasma to anyone because it has no A nor B antibodies in the plasma that will attack the recipient’s plasma.
Can donate to only AB+.

Type AB-

It is the rarest of all blood types — hardly 1% of the adult population has it.
Has both A and B antigens but neither rhesus factor on RBCS nor A or B antibodies in the plasma.
Can donate to only AB+ and AB- blood recipients because it has no A or B antibodies to attack the corresponding antigens of AB blood types.
It is also a universal plasma donor, meaning anyone can receive AB- plasma because it has no A or B antibodies in the plasma that could attack the recipient’s plasma.
Can receive blood from negative types, i.e., A-, AB-, B-, and O-.