ABO and H Antigen Expression

Overview

There are different activity levels of the transferases in converting H antigen into A or B antigen. One significant implication of this is that A2 individuals can create an anti-A1 antibody.

Section Goals

  • Name the lectins used to detect A subgroups
  • Be able to rank ABO blood types in order of H antigen expression
  • Understand why this is important

Why H antigen expression matters

Recall from ABO Antigens – The Basics Are Important that we learned how H antigen is converted into A or B antigen by N-acetylgalactosaminyltransferase or D-galactosyltransferase respectfully. Hopefully these awesome Scrabble words are becoming familiar to you because we’re now going to have some fun learning how ABO type impacts immunohematologic workups.

Group O and H antigen

Group O red blood cells have no A or B antigens, meaning no H antigen has been converted. Therefore, group O has the largest quantity of H antigen and the strongest expression of the H antigen detected in serologic testing. But there are also differences between types A and B and even amongst subgroups of type A.

Group A subgroups and H antigen

Subgroups of a blood type are due to differences in the activities of the transferase enzymes. Some transferases are more “active/efficient” than others and produce slightly different antigens when compared to their peers.

The classic example of this is A1 compared to A2 subgroup. Both A1 and A2 subgroups make A antigen, but their results are a little different.

The differences between A1 and A2 are quantitative, qualitative, and structural.

A1 subgroup

A1 is the most “active/efficient” of the the A subgroups, meaning it is more effective at converting the precursor H antigen into A antigen. It makes more A antigen than other A subgroups. Think of A1 as producing the thick forest pictured here. There is a trunk with lots of thick branches and many leaves. A1 makes a lush antigen forest.

As a result of A1‘s high level of activity in converting H antigen to A antigen, there is less precursor H antigen in A1 individuals compared to other A subgroups.

A2 subgroup

A2 isn’t as “active/efficient” at making antigen compared to A1. To continue the tree metaphor, A2 will make the trunk and a few branches, but it doesn’t produce the thick, lush forest that A1 is capable of making.

Because of this relative reduction in ability to convert H antigen into A antigen, A2 individuals have more H antigen remaining on their red blood cells than A1 individuals.

These differences are important for understanding antibody formation

Because of this structural and quantitative difference, A2 individuals can recognize A1 cells as foreign and mount an antibody response against A1 cells. The A2 individual may see an A1 red blood cell and think “I don’t have all these branches, this is not me.”

However, the A1 cell will recognize A2 as self. The A1 cell may think “I have a trunk just like this, we are the same.” A1 individuals will not make an antibody against A2 cells.

Understanding these differences allows us to appreciate why A2 individuals are capable of making an anti-A1 antibody.

Differentiating A1 from A2

Because of the risk of anti-A1 antibody formation in A2 individuals, we need to differentiate between the two subgroups if testing indicates. Lectins are used to identify A1 vs A2 cells when needed.

The lectin Dolichos Biflorus is used to identify an A1 cell. Dolichos biflorus will be positive with an A1 subtype but will not react with A2 subtypes. 

The lectin Ulex europaeus agglutinates with H antigen and is used to determine the presence of H antigen. Because A2 individuals have more H antigen than A1 individuals, Ulex europaeus lectin results will be positive for agglutination when tested against A2 cells and will be negative in A1 individuals.

In addition, Ulex europaeus can be used to help confirm Bombay Phenotype, as these individuals lack H antigen. A negative result with Ulex europaeus following a pan reactive antibody panel supports the diagnosis of Bombay Phenotype. Review Bombay Phenotype for further details.

Group B and H antigen

In terms of converting H antigen into B antigen, group B’s activity/efficiency is more efficient than A2 subgroup, but less efficient than A1 subgroup.

Quantifying expression of H antigen

I’m hoping it’s become clear that H antigen expression differs between ABO blood types:

O > A2 > B > A2B > A1 > A1B

Why does this matter?

The amount of H antigen expressed can assist in determination of cold autoantibody specificity. I will post some examples shortly, but can you apply the above information to a question:

Question: H antigen expression and autoantibody specificity

A patient who types as group A has an extended antibody panel that is panreactive at 3+ with a autocontrol positive at 1+. The monospecific DAT is positive for IgG and negative for complement. The eluate demonstrates panreactivity at AHG phase. What are your thoughts on why the panel is 3+ but the autocontrol is 1+? (Click for answer)

The group A individual likely has an autoantibody with specificity to H antigen. There is more H antigen present on the group O screening cells than the patient’s group A cells. This is why the screen cells are 3+ positive but the autocontrol is only1+ positive.

Key takeaways:

The key message here is that there are different activity levels of the transferases and the significant effect of this is that A2 individuals can create an anti-A1 antibody. For exam purposes, know that:

  • A1 is more efficient than A2 at converting H antigen into A antigen
  • Both A1 and A2 make A antigen, but there are quantitative, qualitative, and structural differences.
  • H antigen expression differs between ABO blood types: O > A2 > B > A2B > A1 > A1B
  • Dolichos Biflorus is used to identify an A1 cell
  • Ulex eropaeus is used to identify H antigen