Antibody history and antigen-negative units

Try first

A patient's record shows anti-Jk^a identified 3 years ago. Today's antibody screen is nonreactive, and a Jk(a+) red cell unit is compatible through the antiglobulin phase. What can happen if that unit is transfused?

Nothing, because the screen and the crossmatch are both nonreactive

Keep going to see the reasoning.

An immediate hemolytic reaction during the transfusion

Keep going to see the reasoning.

A delayed hemolytic reaction days later, as the antibody returns

Right. Keep going to follow the reasoning.

I don't know yet

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Antibody history and unit selection

Many alloantibodies fall below detection within months to years, and Kidd antibodies do so often.^1,2 The memory B cells that made the antibody remain. Antigen-positive red cells can set off an anamnestic response. The antibody returns within days and shortens the survival of the transfused cells, which is a delayed hemolytic transfusion reaction.^1,3

An antibody is clinically significant when it can shorten the survival of antigen-positive red cells or cause hemolytic disease of the fetus and newborn. Most Rh, Kell, Kidd, Duffy, S and s antibodies are clinically significant.¹ Laboratory policy may allow crossmatch-compatible units for an antibody judged insignificant by validated testing, such as an anti-M that reacts only at room temperature.^1,4

The transfusion service therefore checks the antibody record before every red cell selection:^1,4,5

List every clinically significant antibody ever identified, from today's panel and from the record.
Select red cells that lack every one of those antigens.
Crossmatch through the antiglobulin phase. A current or past clinically significant antibody excludes the patient from the electronic crossmatch.
Keep the antibody on the record for every future specimen.

Estimating how many units to type

Estimated compatible fraction = product of the independent antigen-negative fractions among ABO-compatible units. Units to screen = units needed ÷ compatible fraction, rounded up. Linked antigens and local donor frequencies change the estimate. The fractions multiply because these antigens are inherited independently.¹ Approximate figures for donors of European ancestry:^1,6

Antigen	Donors lacking it
K	91%
E	70%
c	20%
Fy^a	34%
Jk^a	23%
S	45%

Frequencies differ between populations. Most donors of African ancestry lack Fy^a, for example.^1,6 The blood bank's own donor data refine the estimate. Candidate units are typed with antisera, and only the negative ones go on to the crossmatch. When the compatible fraction is very small, the blood supplier can send units already typed for those antigens.¹ The estimate is an average, so be ready to type more units if the first set has too few antigen-negative units.¹

Follow the bench demonstration

Sources for this step

References

Bloch EM, Campbell-Lee S, McKenna DH Jr, Montemayor-Garcia C, Schwartz J, Shaz B, Storry J, eds. Technical Manual. 22nd ed. AABB; 2026.
Hendrickson JE, Eisenbarth SC, Tormey CA. Red blood cell alloimmunization: new findings at the bench and new recommendations for the bedside. Curr Opin Hematol. 2016;23(6):543-549. doi:10.1097/MOH.0000000000000277
Arthur CM, Chonat S, Fasano R, et al. Examining the role of complement in predicting, preventing, and treating hemolytic transfusion reactions. Transfus Med Rev. 2019;33(4):217-224. doi:10.1016/j.tmrv.2019.09.006
Association for the Advancement of Blood & Biotherapies. Fundamental Standards for Blood Collection and Transfusion. 2nd ed. AABB; 2025. Accessed September 27, 2026. https://www.aabb.org/docs/default-source/default-document-library/standards/fundamental-standards-for-blood-collection-and-transfusion-2nd-ed.pdf?sfvrsn=caa34314_3
US Food and Drug Administration. Computer Crossmatch: Computerized Analysis of the Compatibility Between the Donor's Cell Type and the Recipient's Serum or Plasma Type. Guidance for Industry. April 2011. Accessed September 27, 2026. https://www.fda.gov/media/80857/download
Harmening DM, ed. Modern Blood Banking & Transfusion Practices. 7th ed. F.A. Davis; 2019.

Watch one

Read each decision and its explanation.

A woman's record lists anti-Jk^a identified in 2023. She needs 2 units of red cells. Today's panel:

Using the briefing's frequencies, how many ABO-compatible units should be antigen-typed to find 2 suitable units?

Antibody panel: + the cell carries the antigen, 0 it lacks it
Cell	D	C	E	c	e	K	k	Fy^a	Fy^b	Jk^a	Jk^b	M	N	S	s	37	AHG
1	+	+	0	0	+	0	+	+	0	+	0	+	0	+	0	0	0
2	+	+	0	0	+	+	+	0	+	0	+	+	+	0	+	0	0
3	+	0	+	+	0	0	+	+	+	+	+	0	+	+	+	w+	3+
4	0	+	0	+	+	0	+	0	+	0	+	+	+	0	+	0	0
5	0	0	+	+	+	0	+	+	0	+	+	+	0	+	+	0	2+
6	0	0	0	+	+	+	+	+	+	+	0	0	+	+	+	0	0
7	0	0	0	+	+	0	+	0	+	+	+	+	+	0	+	0	0
8	+	0	0	+	+	0	+	+	+	0	+	0	+	+	+	0	0
9	+	0	+	+	+	+	+	0	+	+	0	+	+	0	+	0	2+
10	0	0	0	+	+	0	+	+	0	0	+	+	0	0	+	0	0
11	0	0	0	+	+	0	+	0	+	+	0	0	+	+	0	0	0
Autocontrol																0	0

List the antibodies. Today's panel shows anti-E in cells 3, 5 and 9. The record adds anti-Jk^a.
The historical antibody counts even though the Jk(a+b−) cells 1, 6 and 11 are nonreactive today.
Take the negative fractions: E-negative 0.70, Jk(a−) 0.23.
The estimate uses the donors who lack each antigen.
Multiply: 0.70 × 0.23 = 0.161.
Independent antigens combine by multiplication, so about 16% of donors lack both.
Divide: 2 ÷ 0.161 = 12.4.
Units needed divided by the compatible fraction gives the average number to test.
Round up to 13 units.
Part of a unit cannot be tested, and rounding down plans too few.
Type the units for E and Jk^a, and crossmatch the negative ones through the antiglobulin phase.
The estimate gives the number of units to type. Each unit still needs antigen typing and a crossmatch.

About 13 units: 2 ÷ (0.70 × 0.23) = 2 ÷ 0.161 = 12.4, rounded up to 13.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

A patient's record shows a Kidd antibody identified 2 years ago. Today's antibody screen is nonreactive. Which red cells are selected for transfusion?

ABO-compatible units with an electronic crossmatch

Incorrect. A history of a clinically significant antibody excludes the electronic crossmatch. The patient needs antigen-negative units and a crossmatch through the antiglobulin phase, regardless of today's screen result.

Antigen-negative units with an antiglobulin crossmatch

Correct. Kidd antibodies such as anti-Jka can fall below routine detection and return quickly after re-exposure, shortening donor-cell survival. Units are selected for the recorded antibody even when the current screen is nonreactive.

Crossmatch-compatible units of unknown antigen type

Incorrect. A Kidd antibody below detection can give a compatible crossmatch with antigen-positive cells, which then recruit memory B cells and stimulate an anamnestic response.

Hint

Ask what happens to the cells that made the antibody after the antibody level falls.
Think about what antigen-positive red cells would do to those cells.

Review Anti-Jka with a nonreactive screen

Antibody panel: + the cell carries the antigen, 0 it lacks it
Cell	D	C	E	c	e	K	k	Fy^a	Fy^b	Jk^a	Jk^b	M	N	S	s	AHG
1	+	+	0	0	+	0	+	+	0	+	+	+	0	0	+	2+
2	+	+	0	0	+	+	+	0	+	+	0	+	+	+	0	0
3	+	0	+	+	0	0	+	0	+	0	+	0	+	0	+	0
4	0	+	0	+	+	0	+	+	+	0	+	+	+	+	+	1+
5	0	0	+	+	+	0	+	0	+	+	+	+	0	+	+	0
6	0	0	0	+	+	+	+	0	+	+	0	0	+	0	+	0
7	0	0	0	+	+	0	+	+	0	0	+	+	+	+	+	2+
8	+	0	0	+	+	0	+	0	+	0	+	+	0	+	0	0
9	0	0	0	+	+	0	+	0	+	+	+	+	+	0	+	0
10	+	+	+	+	+	0	+	+	+	+	0	+	+	0	+	1+
11	0	0	0	+	+	0	+	+	+	+	+	0	+	+	+	1+
Autocontrol																0

Problem 2 of 3 · Hints available

A man's panel today identifies anti-Fy^a. His record shows anti-K identified 4 years ago, and the K+ cells 2 and 6 are nonreactive today. Which red cells does he receive?

Fy(a−) units of any K type, because the K+ cells are nonreactive today

Anti-K can fall below detection and return after exposure to K. The historical anti-K still applies, so his units must be K-negative too.

Ignored a historical antibody after a negative screen

Kidd and other antibodies can fall below detection, and the memory B cells that made them remain. Antigen-positive red cells can then trigger an anamnestic response and a delayed hemolytic reaction days after transfusion, so a documented clinically significant antibody still requires antigen-negative units and an antiglobulin crossmatch.

K-negative units of any Fy^a type, because anti-K is the historical antibody

Today's anti-Fy^a is clinically significant too. Units must lack the antigens for every antibody, current and historical.

Fy(a−), K-negative units with an electronic crossmatch

A current or past clinically significant antibody excludes him from the electronic crossmatch. His units need a serologic crossmatch through the antiglobulin phase.

Fy(a−), K-negative units with an antiglobulin crossmatch

Today's anti-Fy^a requires Fy(a−) units, and the historical anti-K requires K-negative units. The antiglobulin crossmatch checks for IgG incompatibility with each unit.

Hint

List every antibody he has ever had, from the panel and from the record.
Ask whether a nonreactive K+ cell today means his anti-K is gone.

Review Unit selection and availability

Problem 3 of 3 · On your own

A patient has anti-K and anti-S and needs 3 units of red cells. The blood bank's donor data show 91% of units lack K and 45% lack S. How many ABO-compatible units should be antigen-typed?

Show the answer

8 units

Compatible fraction = 0.91 × 0.45 = 0.4095. Units to type = 3 ÷ 0.4095 = 7.3, rounded up to 8. Multiplying the antigen-positive fractions by mistake gives 0.09 × 0.55 = 0.0495 and 61 units.

Review Unit selection and availability

Use it

Read the case evidence, then make each decision. The step is complete without it.

For reference

Sources checked September 27, 2026

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Antibody history and antigen-negative units

Try first

Get the idea

Antibody history and unit selection

Estimating how many units to type

Watch one

Your turn

Use it

For reference

The rest of this step has a short briefing, a demonstration at the bench, 3 practice problems and a short case.