12.3 Cell Separations, Neutralization & Thiol Reagents

Cell Separation Techniques

Some workups require physically distinguishing two red cell populations in the same specimen - most commonly the patient's own (autologous) cells mixed with previously transfused donor cells. This matters because a mixed-field reaction on routine typing can otherwise be misread as a weak or discrepant antigen result, and because true phenotype interpretation for antigen-matched transfusion requires knowing which cells are actually the patient's own.

Why donor and patient cells can be separated

Transfused donor red cells are a fixed cohort aging together (up to their ~120-day lifespan), while a patient recovering from hemorrhage or hemolysis is often producing a wave of younger, reticulocyte-rich red cells. This age/density difference is the basis for several separation methods:

• Reticulocyte-harvest methods - a hypotonic wash or a microhematocrit-tube density separation concentrates the lighter, younger reticulocyte-rich fraction (more likely the patient's own regenerating cells) away from the denser, older transfused cohort.
• Differential hemolysis - if patient and donor cells differ in an antigen with a readily available lysin (for example, an ABO-mismatched transfusion, where the other blood type's cells can be selectively lysed with anti-A or anti-B), the unwanted population can be destroyed, leaving the population of interest for testing.
• Density-gradient separation - dextran or similar media separate cell populations by density in a manner similar to reticulocyte harvesting, useful when the age/density difference is the clearest distinguishing feature.

Worked example

A group O patient was transfused two units of group O, Fy(a+) red cells five days ago. A routine antibody screen now shows a mixed-field reaction with anti-Fya antiserum: some cells agglutinate strongly, others do not. Rather than reporting a confusing "weak positive," the technologist recognizes the mixed-field pattern as two red cell populations - donor Fy(a+) cells and the patient's own presumably Fy(a-) cells - and uses a reticulocyte-harvest separation to concentrate the patient's own younger cells before retesting, confirming the patient's true Duffy phenotype rather than reporting a result contaminated by transfused cells.

Neutralization Studies

Neutralization exploits the fact that some blood group antigens exist as soluble substances in body fluids as well as on the red cell membrane. Adding the correct soluble substance to serum before testing will neutralize (bind up) the corresponding antibody, and the antibody's reactivity weakens or disappears - confirming its specificity.

Worked example and caution

A panel shows weak, variable reactions suggestive of anti-Lea. Adding Le(a+) saliva to an aliquot of the patient's serum before retesting causes the reactivity to disappear, confirming anti-Lea. However, neutralization only confirms the presence of that one antibody - it does not rule out a coexisting clinically significant alloantibody hiding in the same serum. In a prenatal patient, for example, a full antibody identification workup must still be completed even after Lewis-antibody neutralization is confirmed, because Lewis antibodies themselves are generally not clinically significant, but a second, significant antibody could still be present.

Thiol Reagents: DTT and 2-Mercaptoethanol

Dithiothreitol (DTT) and 2-mercaptoethanol (2-ME) are reducing agents that cleave disulfide bonds. In the blood bank they are applied in two distinct ways:

1. Treating serum - differentiating IgM from IgG

DTT/2-ME break the disulfide bonds holding IgM pentamers together, converting IgM into non-agglutinating monomeric subunits. Treating serum with DTT and then retesting is a way to determine whether an antibody's reactivity is due to IgM, IgG, or both: reactivity that disappears after DTT treatment was IgM; reactivity that persists indicates an IgG component. This is especially useful in cold agglutinin workups, where a strong IgM autoagglutinin can mask a clinically significant IgG alloantibody.

2. Treating reagent red cells - denaturing Kell and unmasking drug interference

DTT treatment of red cells denatures the Kell glycoprotein, destroying Kell system antigens (K, k, Kpa, Kpb, Jsa, Jsb) while leaving ABO, Rh, Kidd, Lewis, MNS, and Duffy intact. DTT-treated ('Kell-negative-like') reagent cells have a specific, heavily tested application: patients receiving daratumumab, an anti-CD38 monoclonal antibody used to treat multiple myeloma, show panreactive antibody-screen results because daratumumab binds CD38 on every reagent red cell. DTT denatures CD38, eliminating this drug interference and allowing any true underlying alloantibody to be seen. The tradeoff is that DTT-treated cells can never be used to confirm or rule out a Kell-system antibody, since the Kell antigens themselves are destroyed in the process.

ZZAP reagent, mentioned earlier for autoadsorption, is simply DTT combined with a proteolytic enzyme (papain or ficin). The dual action strips bound antibody from the cell membrane (via DTT) while also reducing surface antigen density (via the enzyme), which improves the efficiency of each round of autoadsorption and reduces the number of adsorption cycles needed to remove a strong autoantibody.