Antigens

Chemistry: ABH antigens are glycosphingolipids or glycoproteins.

Development at birth: ABH antigens are not as well developed at birth as they are in adults. Although cord red cells can be ABO grouped, the reactions may be a bit weaker than expected. This is especially true for the A₁ antigen which is very poorly developed at birth: most newborns type as group A₂. In particular, the glycoproteins and glycolipids do not have as many branched chains at birth as are present on adult red cells.

Location in the body: ABH antigens have a very widespread distribution in the body. They are:

1. red cells
2. white cells (in a weakened form)
3. platelets (in a weakened form)
4. body tissue
5. body fluids (if a secretor)

ABH Antigens

Conversion of H antigen by A₁ and A₂ genes

As discussed later, A genes are usually A¹ genes or A² genes. The A¹ gene is a much better converter of H than is the A² gene. Therefore, A₂ red cells have much more H antigen than do A₁ red cells. The amount of H antigen on red cells of the common ABO groups from most to least is O > A₂ > A₂B > B > A₁ > A₁B.

ABH Antigens on Red Cells and in Plasma

A, B, and H immunodominant sugars are added to precursor chains called type 1 and type 2 chains. (See detailed structure of ABH antigens later.) These chains differ in the linkage of a galactose to an N-acetyl-D-glucosamine residue: in type 1 chains the linkage is beta 1 --> 3, and in type 2 chains it is beta 1 --> 4.

Research has shown that the ABH antigens synthesized by the red cells are only carried on type 2 chains and are attached to both glycolipids and glycoproteins. However, some ABH antigens on red cells are acquired from the plasma (and thus are dependent on secretor genes as well as on ABH genes). The ABH antigens acquired from the plasma are on type 1 chains and are glycolipids (glycolipids can be inserted into the membrane of red cells). The site of synthesis of plasma ABH antigens is unknown.

ABH Secretion

Definition: The term secretor, as used in blood banking, refers to secretion of ABH antigens in fluids such as saliva, sweat, tears, semen, and serum. If people are secretors, they will secrete antigens according to their blood groups. For example, group O people will secrete H antigen, group A people will secrete A and H antigens, etc. Soluble (secreted) antigens are called substances.

Inheritance: ABH secretion is controlled by two alleles, Se and se . Se is dominant and se is recessive (or amorphic). Approximately 80% of people are secretors (SeSe or Sese).

Test for Secretor Status (Inhibition Test)

To test for secretor status, an inhibition or neutralization test is done using saliva. The principle of the test is that if ABH antigens are present in a soluble form in a fluid (e.g., saliva) they will neutralize their corresponding antibodies and the antibodies will no longer be able to agglutinate red cells possessing the same antigens. (Note: Secretor status testing is seldom, if ever, done today due to the advent of more potent monoclonal typing reagents and DNA technology.)

Example of Secretor Status Testing (Group A patient)

Applications of the Secretor Status Test

1. to help investigate weak subgroups of A or B. For example, suppose red cells type as follows:

   anti-A	anti-B	A1 cells	B cells
   --	--	--	4+

   This person could be a weak subgroup of A that is not reacting with anti-A. If the person were a secretor, soluble A and H antigens would be present in saliva.

2. In forensic medicine, to investigate a rape case. For example, suppose that an accused rapist is group B, and a semen sample taken from the victim contains soluble H antigen only. The rapist (a secretor) is group O and the accused group B man is NOT the rapist.

Enrichment Activity 1

To obtain practice in solving simple cases of ABO inheritance and in recalling related information, see ABO Exercise "A"

ABH Antigens