AB Detection Reviewer

IMMUNOHEMATOLOGY: ANTIBODY DETECTION — COMPREHENSIVE EXAM REVIEWER

PART 1: ANTIBODY SCREEN

Definition & Purpose

• Used to detect clinically significant antibodies in allogeneic blood donors and patients as part of pretransfusion compatibility testing

• Reagent red cells come with an antigram/antigen profile sheet showing the phenotype of each reagent cell

• A positive antibody screen = unexpected antibody is present in the patient's serum → must perform antibody identification panel

Reagent

• Consists of 2 or 3 group O reagent red cells (2–5%) with known antigen phenotypes

Phases of Reactivity

Interpretation Framework

Step 1: In what phase did the reaction occur?

• IS/Room Temp → IgM, cold-reacting

• AHG phase → IgG, warm-reacting

Step 2: Is the autologous control (autocontrol) negative or positive?

Antibody Screen Interpretation Scenarios

Scenario A — SC I: 2+/neg/1+, SC II: 3+/1+/2+, Auto: neg/neg/neg

• Reactions in both IS and AHG phases, negative autocontrol

• Could represent: multiple warm and cold antibodies, OR a single potent cold alloantibody reacting across phases

• Some IgM antibodies bind complement → detectable in AHG phase even if antibody is IgM (e.g., 2+ IS + 1+ AHG = potent cold alloantibody with complement binding)

Scenario B — SC I: neg/neg/1+, SC II: neg/neg/1+, Auto: neg/neg/neg

• Both screening cells reactive only in AHG, autocontrol negative → possible interpretations:

  • Antibody directed against a high prevalence antigen (present on nearly all RBCs)

  • Complement-fixing cold alloantibody

  • Single warm alloantibody (antigen present on both screening cells)

Scenario C — SC I: neg/neg/3+, SC II: neg/neg/3+, Auto: neg/neg/3+

• All reactive including autocontrol in AHG phase → suggests warm autoantibody

• In recently transfused patients (within 3 months) → may indicate donor RBCs coated with alloantibodies mimicking warm autoantibody → resembles autoimmune hemolytic anemia on panel

• Could also represent a transfusion reaction, or IgG alloantibody coexisting with a warm autoantibody

A Positive Antibody Screen Requires:

1. Compatibility testing

2. Antibody identification

PART 2: ANTIBODY IDENTIFICATION

Definition

• Process of determining the specific antibody or antibodies present after a positive antibody screen

• Only performed when antibody screen is positive

• Antibody screen = first-tier test (detects presence); Antibody ID = second-tier test (determines specificity)

• Critical because it directly influences transfusion safety (e.g., patient with Anti-K must receive K-negative blood)

• Rule: If you cannot identify the antibody, you cannot safely match the blood.

Antibody Identification Panel

• Consists of 10–20 group O reagent red cells with known phenotypes

• Required antigens: D, C, E, c, e, M, N, S, s, P1, Le(a), Le(b), K, k, Fy(a), Fy(b), Jk(a), Jk(b)

• "+" = antigen present on cell; "0" = antigen absent

PART 3: EXCLUSION / "RULE-OUTS"

Definition

• Examine RBCs that gave a negative reaction in all phases

• The antigens on negatively reacting cells are probably not the antibody's target

• Rule-out is only valid with homozygous expression of the antigen on that cell

Homozygous vs. Heterozygous

Antibodies That Show Dosage Effect

(Mnemonic: Rich Kidd Duffy may MeNS Lagi — excludes D from Rh)

• Rh: anti-C, anti-c, anti-E, anti-e

• MNS: anti-M, anti-N, anti-S, anti-s

• Duffy: anti-Fy(a), anti-Fy(b)

• Kidd: anti-Jk(a), anti-Jk(b)

Exceptions — Can Be Ruled Out Even if Heterozygous

(Low prevalence antigens rarely expressed homozygously)

• anti-K

• anti-Kp(a)

• anti-Js(a) (Sutter)

• anti-Lu(a)

Rule-Out Logic Summary

Rule of 3

• Positive side: Need at least 3 positive reactive cells

• Negative side: Need at least 3 negative reactive cells

• If fewer than 3 positives → antibody is strongly suggested but cannot be reported until a third positive cell is found

PART 4: SPECIAL REAGENTS — FICIN

• Ficin = a proteolytic enzyme derived from fig (Ficus carca)

• Treats red cells to modify antigen expression:

• Use: If reactions occur only on ficin-treated cells and Duffy/MNS are destroyed → antibody must target Rh, Kidd, or Lewis systems

PART 5: PANEL INTERPRETATION — STEP-BY-STEP

Step 1 — Identify Reactive Cells

• Note which cells are positive on IAT/AHG (or IS/37°C if applicable)

• Check autocontrol → negative = alloantibody; positive = autoantibody or recently transfused

Step 2 — Rule Out Non-Reactive Cells

• For each non-reactive cell, cross out any antigen that is homozygously expressed (+ for that antigen, 0 for its partner)

• Leave heterozygous cells (+/+ for both antigens) alone

Step 3 — Check Reactive Cells

• Of the antigens remaining (not crossed out), check which are present (+) on the reactive cells

• The surviving antigen present on all reactive cells = candidate antibody

Step 4 — Confirm with Rule of 3

• ≥ 3 reactive cells expressing the antigen = confirms positive side

• ≥ 3 non-reactive cells lacking the antigen = confirms negative side

PART 6: WORKED CASE SUMMARIES

Example 1 — Anti-K

• Gel (IAT), reactive cells: 7 and 11; autocontrol negative → alloantibody

• After rule-outs: Kell (K) is the only antigen not eliminated

• Kell antibodies are warm-reacting → IgG

• Result: Anti-K, alloantibody, IgG (Rule of 3 not yet met — needs one more K+ reactive cell)

Example 2 — Anti-E

• Reactive cells: 3 and 6; reaction in all 3 phases but strengthens IS → AHG → IgG alloantibody (not cold)

• Cell 3 (R2R2 = DcE/DcE): homozygous E → strong agglutination at all phases

• Cell 6 (r2r = cE/ce): heterozygous E → weaker agglutination (dosage effect)

• Result: Anti-E, warm-reacting, alloantibody

Example 3 — Anti-C (Ficin Panel)

• Ficin used → Duffy and MNS destroyed and ruled out automatically

• Reactive cells: 1, 2, 5, 11

• After rule-outs: only the C antigen (Rh) remains unexplained

• Result: Anti-C, identified using ficin-treated panels

PART 7: CLINICAL CASE SUMMARIES

Case 1 — Anti-D

• 29 y.o. Rh-negative pregnant female (G4P3), no transfusion history

• AHG: SC I = 1+, SC II = 1+, SC III = 0; autocontrol negative

• Panel cells 1, 2, 3, 4, 11 positive; 5–10 and AC negative → pattern matches D antigen column

• Sensitized from prior pregnancies (fetal D+ RBCs crossing placenta)

• Clinically significant — can cause HDFN (Hemolytic Disease of the Fetus and Newborn)

• Result: Anti-D, warm-reacting (IgG), alloantibody

Case 2 — Anti-Fya

• 69 y.o. male, 16 units PRBCs transfused (most recent 6 months ago)

• AHG: SC I = 1+, SC III = 3+; autocontrol negative

• Panel cells 1, 2, 6, 8, 9, 10 positive → pattern matches Duffy A (Fya) column

• Varying reaction strength (1+ vs 3+) = dosage effect (Duffy shows dosage)

• Sensitized from multiple transfusions

• Result: Anti-Fya, alloantibody

Case 3 — Suspected Anti-Jkb (Complex Panel)

• 47 y.o. female, 47 RBC transfusions (most recent 8 days ago)

• DAT: Polyspecific 1+ mf, IgG 2+ mf, C3d 1+ mf → RBCs coated with IgG and complement

• Autocontrol positive (1+ mixed-field) → not a clean panel

• Pattern suggests anti-Jkb, BUT positive autocontrol + recent transfusion → donor Jkb+ RBCs being attacked by patient's anti-Jkb

• Result: Not cleanly interpretable; requires autoadsorption studies + panel retesting

Case 4 — Anti-Fya + Anti-K (Multiple Antibodies)

• 64 y.o. male, 30 units PRBCs

• Panel cells 1, 2, 6, 7, 8, 9, 10, 11 positive; 3, 4, 5, AC negative

• Fya column matches cells 1, 2, 6, 8, 9, 10 but cells 7 and 11 remain unexplained

• K column positive for cells 7, 10, 11 → explains the remainder

• Cell 10 is both Fya+ and K+ → reacts strongest (4+)

• Result: Anti-Fya + Anti-K (multiple alloantibodies)

Case 5 — Anti-M (Cold, IS Only)

• 28 y.o. Rh-negative primigravida, prenatal workup

• A1 cells = 1+ → possible A2 subgroup with anti-A1

• IS: SC I = 2+, SC II = 2+, SC III = 0; AHG: all negative

• Reaction only at IS, disappears at AHG → cold-reactive IgM antibody

• IS pattern matches M antigen column

• Result: Anti-M, cold-reactive IgM (often not clinically significant unless reactive at 37°C/AHG)

QUICK REFERENCE SUMMARY TABLE

Key principle to remember for exams: Antibody screen detects the presence of an antibody. Antibody identification names it. You cannot give compatible blood until you name it.