Extra reactivity in cell grouping tests ("extra antigens") refers to unexpected positive results in the front cell group as shown in Table 4-8. Three causes of extra antigens are discussed below.

Table 4-8. "Extra antigens."

1. Acquired B Phenomenon: Example #1 could be an example of acquired B phenomenon.

   Mechanism/Cause: there are two causes of acquired B phenomenon:

   1. In vivo, patients with bacterial infections and often cancer of the colon or rectum may develop a false B-like antigen. The actual cause is the bacterial infection; the Ca of the colon or rectum merely allows bacterial products to enter the blood stream more easily. The mechanism by which the bacteria cause red cells to acquire a B-like antigen is as follows: The bacterial produce a deacetylase (enzyme) which chemically alters the terminal sugar of A antigens (N-acetyl-D-galactosamine) into D-galactosamine. Because the terminal sugar of the B antigen is galactose, anti-B antisera will cross react with the B-like D-galactosamine antigen. Because of this, in vivo, only group A people can develop an acquired B-like antigen. The condition is transient and disappears when the infection is cured.

      The use of monoclonal ABO typing antisera (specifically an anti-B clone designated "ES4") initially caused an increase in acquired B phenomenon, because the ES4 monoclonals can detect even a small number of galactosamine molecules on red cells. However, the reactions are particularly sensitive to pH and can be reduced (not eliminated totally) if the pH is lowered, something that the manufacturers have done.

   2. In vitro, blood specimens can get an acquired B-like antigen if they are bacterially contaminated. This is because the membranes of some bacteria (e.g., E. coli and P. vulgaris ) have determinants which are chemically similar to the B antigen. In this case, anti-B antisera is actually reacting with the bacterial antigens which have attached to the red cells. In vitro, both group O and group A cells can acquire the B-like antigen. Note: most examples of acquired B phenomenon detected in the blood bank happen in vivo to group A people only.

   Typical reaction pattern:

   Resolution:

   Implications for blood transfusions:

2. Polyagglutination due to T-Activation

   Example #1, #2, and #3 could be examples of T-activated red cells.

   Mechanism/Cause: T-activation is caused by bacteria or viruses that produce the enzyme neuraminidase. All normal red cells have a T antigen that is hidden or covered up by NeuNac (NANA or sialic acid) residues, and all normal sera contain anti-T, an IgM antibody usually reactive at RT or lower. Patients with a bacterial or viral infections that produce neuraminidase can have their T antigen exposed or "activated." Their red cells then will become polyagglutinable because almost all sera (including ABO typing sera) contain anti-T. The condition is transient in vivo . (It can also happen via the same mechanism in vitro if specimens become bacterially contaminated).

   Typical reaction pattern:

   Resolution:

   Implications for Transfusion:

3. Wharton's Jelly

   Example #4 could be a cord sample from a newborn. Wharton's jelly is the jelly-like substance present in the umbilical cord. If cord blood samples are improperly drawn, i.e., cut and "milked," Wharton's jelly will be present. Although not really an extra antigen, Wharton's jelly can mimic extra antigens.

   Typical reaction pattern:

   Resolution:

Read HTR due to acquired B , a Medline abstract of a case report in which acquired B phenomenon resulted in a fatal hemolytic transfusion reaction (HTR).

Answer the following questions based on the abstract and e-mail replies to Pat.

1. Briefly discuss what this case illustrates about the reliability of using the reverse serum group to confirm the front cell group.

2. The immediate spin RT crossmatch is designed to ensure ABO compatibility between patient and donor. Discuss its reliability.

3. Discuss whether an electronic crossmatch would have prevented the hemolytic transfusion reaction.

4. Would an IAT crossmatch have prevented the transfusion reaction?

5. Many labs use monoclonal ABO typing sera. Do they have perfect sensitivity or specificity? What precautions are needed when using them?

Extra Antigens