16.3 Thiol Reagents, Neutralization, Chloroquine, EGA, and Immunofluorescence

16.3 Thiol Reagents, Neutralization, Chloroquine, EGA, and Immunofluorescence

Quick Answer: DTT and 2-ME reduce disulfides. On serum, they destroy IgM pentamers so remaining activity is IgG. On cells, 0.2 M DTT destroys Kell, Lutheran, Dombrock, and JMH (and CD38). Neutralize Lewis, P1, Ch/Rg, and Sda with the matching soluble substance. Chloroquine diphosphate strips IgG from DAT-positive cells so you can phenotype; it may weaken Rh and spares Kell. EGA (EDTA-glycine acid) also strips IgG but destroys Kell — do not type Kell after EGA. Immunofluorescence / flow quantify FMH and detect platelet antibodies.

The June 9, 2026 outline lists thiol reagents at IV.C.11, immunofluorescence at IV.C.12, chloroquine diphosphate at IV.C.15, and EDTA-glycine acid at IV.C.16. Chapter 14 already used DTT as a panel trick and neutralization as an HTLA move. This section is the reagent itself: what it destroys, what it spares, and which bottle you reach for when the DAT is 3+ and the floor wants a phenotype.

Thiol reagents: DTT and 2-ME on serum versus on cells

Dithiothreitol (DTT) and 2-mercaptoethanol (2-ME) reduce disulfide bonds. They are not enzymes and they are not chloroquine. The same chemistry does two different jobs depending on what you treat.

Treat the serum or plasma when the question is IgM versus IgG. The IgM pentamer (and J chain) falls apart; saline-reactive IgM activity disappears. IgG monomers remain. That is how you decide whether a cold agglutinin, an anti-M, or an ABO antibody in a prenatal patient is still dangerous at 37 °C as IgG. Trap: DTT also destroys Kell antigens on any cells still in that mixture, and anti-K can look like “it was IgM” if you treated a cell suspension instead of cell-free serum. Destroying anti-K is not proof of IgM.

Treat the reagent cells (typically 0.2 M DTT) when you need an antigen-destroyed panel. Disulfide-dependent structures fall:

AET is a related thiol used historically to make Kell-null-like cells. ZZAP is DTT plus a protease — the autoadsorption reagent that strips IgG and destroys both Kell and the enzyme-sensitive antigens. Do not use a ZZAP-treated autocontrol cell to phenotype Kell or Duffy.

Chapter 8 already gave you the fingerprint: enzymes spare Kell; DTT destroys Kell. Chapter 14 gave you the daratumumab sentence: DTT-treated panel cells clear CD38 interference and take Kell with them, so you still issue K-negative units or type K another way.

Neutralization

Neutralization adds a soluble antigen to plasma so one antibody is consumed and whatever remains can be identified.

• Lewis: saliva from a Lewis secretor, or commercial Lewis substance.
• P1: hydatid-cyst fluid, pigeon-egg white, or commercial P1 substance.
• Ch/Rg: plasma (they are adsorbed C4).
• Sda: urine (Tamm–Horsfall glycoprotein).
• ABH: secretor saliva, when you are proving an ABO antibody rather than typing cells.

Always run a saline or diluent control in parallel so a weaker leftover reaction is not just dilution. Neutralization is how you prove an HTLA-like anti-Ch and then look underneath for anti-Jka. It does not destroy Kell, and it is not a substitute for DTT.

Chloroquine diphosphate versus EGA

Both reagents dissociate coating IgG so DAT-positive cells can be phenotyped. They are not interchangeable.

Chloroquine diphosphate removes much of the bound IgG without destroying most blood-group antigens. After treatment, the DAT should be negative (or markedly weaker) before you trust the phenotype. Rh antigens, especially D, may weaken — a true D-positive can look D-negative or weak D after chloroquine. Repeat D typing with an appropriate control, or genotype if the result will change RhIG or unit selection. Kell survives chloroquine. That is the reason to pick this bottle when the question is “is this DAT-positive patient K-negative?” Chloroquine can also strip some Bg (HLA) reactivity from reagent cells. Strongly coated warm-auto cells may not strip completely; residual DAT-positive cells are still invalid for typing.

EDTA-glycine acid (EGA) is a rapid acid method that strips IgG very effectively, including coatings chloroquine left behind. The cost is specific: EGA destroys Kell-system antigens. After EGA you cannot report K, k, Kpa, Kpb, Jsa, or Jsb. EGA is a good choice when you need an Rh (or other non-Kell) phenotype on heavily coated cells and Kell can wait for genotyping or a different dissociation. EGA is chemically related to acid elution — the supernatant can be saved as an eluate in some protocols, but the exam point is the treated cell, not a replacement for a formal acid eluate workup (Chapter 15).

When to pick which:

• Need Kell phenotype on DAT-positive cells → chloroquine (or gentle heat), not EGA.
• Chloroquine failed, residual DAT+, need Rh/C/c/E/e → EGA, then genotype Kell (and anything else EGA ruined).
• Need both Kell and Rh and chloroquine left a positive DAT → do not guess; genotype.
• Never phenotype Kell after EGA. That is the named trap.

Neither reagent is a substitute for elution when you need the specificity of the coating antibody. Strip to type the cell; elute to name what came off.

Immunofluorescence and flow cytometry

IV.C.12 is not a red-cell panel. Labeled antibodies plus a fluorescence readout answer two BB questions.

Feto-maternal hemorrhage: flow cytometry using anti-HbF and/or anti-D enumerates fetal red cells more precisely than a Kleihauer–Betke stain. Use it to quantify a bleed so RhIG can be dosed (Chapter 12). Skip a D-dependent method (rosette, anti-D-only flow) when the infant is weak D; use anti-HbF. Flow does not replace the qualitative rosette screen in every laboratory, but it is the better quantitative assay when it is available.

Platelet antibodies: the platelet immunofluorescence test (PIFT) and platelet flow cytometry detect IgG bound to platelets (NAIT, PTP, immune refractoriness). Granulocyte immunofluorescence (GIFT) is the HNA analog. These assays live next to MAIPA and HLA PRA. They do not belong on an RBC antibody panel, and DTT or EGA on red cells will not answer a platelet-refractory workup.

Worked scenario. A warm-auto patient needs K and e types. The DAT is 3+ IgG. Chloroquine brings the DAT to 0; e types 3+ and K types 0. Report K-negative, e-positive, and honor that for unit selection. If you had used EGA first, a K-negative result would be uninterpretable — the antigen may have been destroyed rather than absent.

Exam traps

• Treating serum with DTT and concluding anti-K was IgM because reactivity vanished — you may have destroyed Kell antigen on leftover cells.
• Ruling out anti-K on a DTT-treated panel (including a daratumumab workup).
• Phenotyping Kell after EGA.
• Calling a patient D-negative after chloroquine without considering Rh weakening.
• Using EGA when the only question is the K type.
• Treating flow/immunofluorescence as a replacement for the antibody screen instead of as FMH quantitation or platelet serology.
• Skipping the dilution control on a neutralization and calling the antibody gone.