ABO Antibodies
Overview
ABO antibodies are crucial to understand well. Here we’ll cover how ABO antibodies are formed and review blood group compatibility.
Section Goals
- Explain the difference between naturally occurring and immune mediated red blood cell antibodies
- Understand when individuals begin producing their own ABO antibodies
- Be able to identify both antigens and antibodies present for each ABO type
- Understand the difference between an auto anti-H antibody from an allo anti-H antibody
- Know what types of RBCs and Plasma are compatible for each ABO type
ABO antibodies are naturally occurring
Red blood cell antibodies, also called isoagglutinins, are part of the humoral immune response; they are made by plasma cells in response to exposure to foreign red cell antigens. Exposure can be naturally occurring or immune mediated.
Naturally occurring red blood cell antibodies are the result of interaction with our environment. Pollen and intestinal flora can contain substances similar to A and B antigens and can trigger an immune response in individuals lacking these antigens. Contrast this with immune mediated red blood cell antibodies, which are formed following exposure to foreign red cell antigens, commonly via transfusion or pregnancy. We’ll discuss immune mediated red blood cell antibodies further in a later section.
Individuals develop naturally occurring ABO antibodies during their first year of life. Environmental exposure occurs and antibody production begins with maturation of the immune system, usually after four months. This is why we do not test for ABO antibodies in neonates until the age of four months. But understand that neonates under four months old may still have ABO antibodies present due to passive transfer from the mother across the placenta. The ABO antibodies we produce independently are mostly IgM, with a smaller amount of IgG. Recall that IgM does not cross the placenta due to its large pentameric structure. IgG antibody is capable of crossing the placenta. This will be important when we discuss hemolytic disease of the fetus and newborn (HDFN), but for now simply understand that testing for antibodies prior to 4 months of age will only result in identifying the mother’s isoagglutinins.
ABO antibodies continue to develop and increase in strength as we grow, reaching full strength by 5-10 years of age and persisting until old age, at which time these antibodies tend to diminish in strength.
ABO antibodies by blood group
Understanding that we make ABO antibodies to the antigens we lack makes it easier to define the antibodies by blood group. Though we learn this in medical school, now is the time to make this information second nature:
| ABO Type | ABO Antigens Present | ABO Antigens Produced |
|---|---|---|
| A | A, H | Anti-B |
| B | B, H | Anti-A |
| O | H | Anti-A, Anti-B, Anti-A,B |
| AB | A, B, H | None |
A final word on anti-H
We’ve discussed anti-H quite a bit so far. If you need, refresh yourself by revisiting ABO and H Antigen Expression and Bombay Phenotype. Now, lets distinguish between the two types of anti-H that can occur.
There is the autoantibody version of anti-H. This is found in individuals that have the H antigen present on their red cells. It is often a cold autoantibody, meaning it reacts best at room temperature or colder and is often IgM. In fact, cold autoantibodies often have H antigen specificity. Recall that group O red cells have the most H antigen present since it is not converted to A or B antigen. Therefore, a cold auto anti-H antibody often reacts strongest with type O red cells, and the antibody identification panel will reflect this. If the patient is non group O then their autocontrol is typically weaker than the panel cells. See the example at ABO and H Antigen Expression for review if needed. Because it is a cold reactive antibody, auto anti-H is rarely clinically significant since our bodies rarely cool to a temperature at which the antibody is active.
Bombay Phenotype results in an alloantibody version of anti-H. This is a naturally occurring alloantibody formed by individuals lacking the H antigen. The alloantibody has both IgM and IgG forms. Because IgG has an optimal reactive temperature of 37 degrees, this anti-H is considered clinically significant because it can cause hemolysis. Therefore, a Bombay individual can only receive blood that lacks the H-antigen (i.e. from another Bombay individual). Review Bombay Phenotype if necessary.
Blood Product Compatibility
If you’ve grasped the above concept of ABO antibodies by blood group, understanding compatibility will come naturally.
RBC compatibility is defined by the antibodies in the recipient’s plasma. Avoid giving RBCs that will be attacked by the recipient. For example, a group A recipient has an anti-B antibody. Avoid giving group B or AB red blood cells to this recipient.
Plasma compatibility is defined by the antibodies present in the donor’s plasma. Avoid giving plasma that contains antibodies that will attack the recipient’s cell. For example, our group A recipient has A antigen. Group O and group B plasma contains an anti A. Therefore, avoid giving these plasma products to this recipient.
| Recipient Blood Type | Compatible RBCs | Compatible Plasma |
|---|---|---|
| A | A, O | A, AB |
| B | B, O | B, AB |
| O | O | O, A, B, AB |
| AB | AB, A, B, O | AB |
When learning these for the first time, remind yourself to slow down and think it through.
Universal Red Cell Donor
Anyone can receive type O red cells. Because of this, type O donors are the universal red cell donors.
Universal Plasma Donor
Because AB plasma it does not contain any ABO antibodies, anyone can receive AB plasma. Type AB donors are the universal plasma donors.
Key takeaways:
Hold
- ABO antibodies are naturally occurring red blood cell antibodies stimulated by exposure to the world around us (pollen and intestinal flora)
- Immune mediated red blood cell antibodies form following direct exposure to foreign red cell antigens (via transfusion or pregnancy)
- Cold autoantibodies often have specificity for the H antigen and are usually NOT clinically significant
- Bombay Phenotype produces an allo anti-H antibody, which is clinically significant
- Type O donors are the universal red cell donors
- Type AB donors are the universal plasma donors