14.2 Clinical Significance, Dosage, and Multiple Antibodies

14.2 Clinical Significance, Dosage, and Multiple Antibodies

Quick Answer: Always honor antibodies to ABO, Rh, Kell, Duffy, Kidd, S/s/U, and P/PP1Pk. Usually ignore (for red-cell selection) Lewis, I, P1, and M/N unless they react at 37 °C / AHG, plus most HTLA antibodies (Knops, Ch/Rg). Dosage is the hallmark of Kidd, Duffy, Rh, and MNS. Multiple antibodies look like leftover reactions after one specificity is assigned. Separate them with selected cells and an enzyme panel: Rh and Kidd enhanced, Duffy and MNS destroyed, Kell unaffected. Warm auto plus allo needs differential adsorption. All cells positive, AC negative is high-prevalence until you prove multiple alloantibodies.

Identification is not finished when you can pronounce a name. IV.A.3 also wants the clinical decision: which antibodies get antigen-negative units, which ones you prewarm away, and how a second specificity hides in the leftovers.

Always significant versus usually insignificant

Clinically significant means the antibody can cause a hemolytic transfusion reaction or HDFN and must be honored with antigen-negative, AHG-crossmatch-compatible units.

Always significant — treat as dangerous even when the reaction is only 1+:

• ABO — anti-A, anti-B, anti-A,B. Intravascular hemolysis if you get this wrong.
• Rh — anti-D, -C, -c, -E, -e, and the rest of the system once immune.
• Kell — anti-K and the high-prevalence Kell antibodies (k, Kpb, Jsb, Ku).
• Duffy — anti-Fya, anti-Fyb, and anti-Fy3 in Fy(a−b−) people.
• Kidd — anti-Jka, anti-Jkb, anti-Jk3. Infamous for delayed extravascular hemolysis and for dropping below detectability between transfusions.
• S, s, U — the IgG MNS antibodies. Anti-U is a high-prevalence problem in S−s− people of African ancestry.
• P and PP1Pk — alloanti-P (P1k phenotype) and anti-PP1Pk (p phenotype, historically anti-Tja) cause severe HTR and early miscarriage / HDFN.

Usually insignificant for red-cell selection if they are not active at 37 °C / AHG:

• Lewis (Lea, Leb) — typically IgM, neutralize with saliva, poorly expressed on cord cells.
• I — cold autoanti-I is the adult-cell cold agglutinin; not an alloantibody you honor for units.
• P1 — cold, often IgM; neutralize with P1 substance.
• M and N — honor only if they react at 37 °C or AHG (then they can be IgG and can cause HTR/HDFN). Room-temperature-only anti-M is prewarmed away and ignored.
• HTLA antibodies — Knops (Kn, McC, Sl, Yk), Chido/Rogers (Ch, Rg — C4 antigens on red cells), Cost, and some JMH. Weak, variable, high titer, low avidity. Not associated with hemolysis.

The exam trap is anti-M that is still 2+ at AHG. That one is not “usually insignificant.” Honor it. The same sentence applies to a Lewis antibody that refuses to disappear at 37 °C — uncommon, but the phase wins over the system’s reputation. Conversely, do not antigen-type a fridge full of Le(a−) units for an IS-only anti-Le(a).

Kidd deserves a second sentence. Anti-Jka that is undetectable today is still an antibody you honor forever once it has been identified. Issue Jk(a−) units on the historical record even if this morning’s screen is clean.

Dosage is a property of the antigen, not a suggestion

Dosage means a double-dose (homozygous) cell reacts stronger than a single-dose (heterozygous) cell. Sometimes the heterozygous cell is nonreactive. That is how Kidd antibodies sneak through a two-cell screen that happened to use Jk(a+b+) cells.

Systems that show dosage and therefore require homozygous rule-out:

• Kidd — the prototype. Anti-Jka may be 3+ with Jk(a+b−) and 0–1+ with Jk(a+b+). Delayed HTRs happen when the antibody is undetectable and the patient receives a Jk(a+) unit.
• Duffy — anti-Fya is stronger with Fy(a+b−) than Fy(a+b+).
• Rh — especially anti-c, anti-E, anti-C, anti-e. An R1R2 (C+c+ E+e+) cell is a terrible rule-out for anti-c or anti-E.
• MNS — anti-M, anti-N, anti-S, anti-s all dosage. Anti-M classically prefers MM cells at colder phases.

Kell does not show useful dosage. Do not wait for a rare K+k− cell to exclude anti-K. A nonreactive K+k+ cell is enough.

When two antibodies are present, dosage on one of them can look like a second pattern. A 3+/1+/0 panel is not automatically “two antibodies” — check whether the 3+ cells are double-dose and the 1+ cells are single-dose for one antigen (the Section 14.1 anti-Jka mini-panel). If the 1+ leftovers do not share that antigen, then you really do have a mixture.

Multiple antibodies: leftover reactions

Assign the clearest specificity first. Then look at the cells that are negative for that antigen and still reactive. Those leftovers are the second antibody.

Example: every E+ cell is 2+, but two E− cells are also 2+, and both of those E− cells are K+. The E− K− cells are 0. That is anti-E plus anti-K, not “anti-E that sometimes reacts.” Selected cells: E− K+ (should react) and E− K− homozygous for anything still not excluded (should not).

If leftovers do not share one antigen, you may have three antibodies or a low-frequency antigen on one cell. Do not invent a third antibody to explain a single extra 1+ — check that cell's Kpa, Jsa, Lua, Cwa, Wra. One unmatched low-frequency reaction is a footnote, not a new common alloantibody.

A mixture of three common antibodies (anti-c + anti-E + anti-K, or anti-Fya + anti-Jka + anti-S) can leave almost no negative cells. That is when people confuse the picture with a high-prevalence antibody. The difference: multiple common alloantibodies almost always leave at least one panel cell negative if you look, and the patient types negative for several common antigens. A true high-prevalence antibody leaves no common panel cell negative, and the patient is missing one high-prevalence antigen.

Enzyme panel as a separator

A ficin or papain panel is the fastest bench tool for mixtures.

Worked separations:

• Anti-E + anti-Fya: untreated panel looks messy. The enzyme panel keeps only the E+ reactions (Rh enhanced, Fya destroyed). The Fya component vanishes.
• Anti-c + anti-Jka: both enhanced. Enzymes will not separate them. Use selected c− Jk(a+) and c+ Jk(a−) cells.
• Anti-K + anti-Fya: enzymes kill Fya and leave K (Kell unaffected).
• Anti-Jka + anti-S: enzymes enhance Kidd and destroy S, so the leftover enzyme-panel pattern is the Kidd antibody.

Write the enzyme table on the inside of your skull. The exam loves a stem that says “reactivity was abolished by ficin” or “reactions were stronger with papain-treated cells.”

Warm auto plus allo, and high-prevalence versus a pile of alloantibodies

A warm autoantibody reacts with all panel cells and the autocontrol at AHG. It can hide an alloantibody. If the patient has not been transfused in 3 months, perform autologous adsorption (ZZAP- or enzyme-treated autologous cells) and test the adsorbed plasma for leftover alloantibody. If the patient was recently transfused, autologous cells are contaminated with donor cells that might pull out the very alloantibody you are hunting. Use differential (allogeneic) adsorption: three adsorbing cells of known phenotype, typically R1R1, R2R2, and rr, chosen so that each adsorbs the auto and a different slice of common antigens. Antibody left behind in one adsorbed aliquot is the allo.

All panel cells positive, autocontrol negative is not a warm auto. That is a high-prevalence alloantibody (anti-k, anti-Kpb, anti-Jsb, anti-U, anti-PP1Pk, anti-Vel, anti-Lub, anti-Yta) or so many alloantibodies that no panel cell is negative for all of them. Separate those two with:

• Patient phenotype/genotype — a U−, k−, or Vel− type names the suspect.
• One negative cell of a rare phenotype.
• Enzymes and DTT (Section 14.3) to put the antibody in a system.
• IRL if you cannot find a negative cell.

Multiple common alloantibodies almost always leave some panel cells negative. A true panagglutinin with a negative AC is high-prevalence until proven otherwise.

Exam traps

• Issuing random units for anti-Jka because the current screen is negative. Kidd disappears and comes back as a delayed HTR.
• Treating AHG-reactive anti-M like room-temperature anti-M.
• Ruling out anti-c on an R1R2 cell.
• Calling a panagglutinin with a negative AC a warm auto.
• Using autologous adsorption on a patient transfused 10 days ago.
• Expecting enzymes to separate anti-E from anti-Jka — both are enhanced.