Significance of minor red blood cell antibodies during pregnancy

NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.

Dean L. Blood Groups and Red Cell Antigens [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2005.

Blood Groups and Red Cell Antigens [Internet].

Chapter 10 The Kidd blood group

The Kidd (JK) glycoprotein is the red blood cell (RBC) urea transporter. Situated in the membrane it rapidly transports urea into and out of RBCs, maintaining the osmotic stability and shape of the RBC in the process. The Kidd glycoprotein is also expressed in the kidney, where it enables the kidney to build up a high concentration of urea which is needed for the kidney to produce concentrated urine.

People who do not produce the Kidd glycoprotein tend not to be able to maximally concentrate urine, but despite this, they are healthy and their RBCs have a normal shape and lifespan.

Antibodies that target Kidd antigens are a significant cause of delayed hemolytic transfusion reactions. Anti-Kidd antibodies are also a cause of hemolytic disease of the newborn (HDN), the severity of the disease varies but tends to be mild in nature.

At a glance

Antigens of the Kidd blood group

Amino acid sequence determines the specificity of Kidd antigens

The Kidd protein is a transmembrane, multi-pass protein that transports urea across the RBC membrane.

Located on chromosome 18 (18q11-q12), contains 11 exons that span more than 30 kbp of DNA. The SLC14A1 gene has two major codominant alleles, Jk a and Jk b , which result from a SNP (838G→A), and the corresponding Jk a and Jk b antigens differ by a single amino acid (D280N).

Jk b : 74% Caucasians, 49% Blacks, and 76% Asians

Jk3: 100% in most populations, >99% in Polynesians (1)

Jk(a+b-): 26% Caucasians, 51% Blacks, 23% Asians

Jk(a-b+): 23% Caucasians, 8% Blacks, 27% Asians

JK(a-b-): Rare in most populations, found in 0.9% Polynesians (1)

Antibodies produced against Kidd antigens

IgG is more common

Can bind complement

Anti-Jk a and anti-Jk b are dangerous antibodies because they can be difficult to detect in routine blood cross-matches. They are a common cause of delayed hemolytic transfusion reactions. Anti-Jk3 is rare and can cause immediate and delayed hemolytic transfusion reactions.

Anti-Jk a has been implicated in at least one severe case of HDN, but most cases of HDN caused by the anti-Kidd antibodies are mild in nature.

Background information

In 1951, a patient called Mrs. Kidd was found to have produced antibodies targeted against a then unknown red cell antigen during her pregnancy. The marker was present on the RBCs of her fetus, and the maternal antibodies targeted against it caused fatal hemolytic disease in her newborn child.

The protein was given the name Jk a and was the first antigen to be discovered in the Kidd blood group system. Since this time, two other antigens, Jk b and Jk3, have been found.

In 1959, the first example of the null phenotype, i.e., Jk(a-b-), was found in a woman who had become jaundiced after a blood transfusion. Her serum was found to contain an antibody that recognized both Jk a and Jk b . This antibody was subsequently named anti-Jk3.


Basic biochemistry


There are three common Kidd phenotypes: JK(a+b-), JK(a-b+), and JK(a+b+).

The Jk-null phenotype, JK(a-b-), is rare in most populations. Individuals with this blood type are often detected after they have been immunized to Kidd antigens during a previous blood transfusion or pregnancy. After immunization, JK(a-b-) individuals form anti-Jk3, which can cause HDN in subsequent pregnancies and hemolyse donor blood that contains Jk a and/or Jk b antigens during a subsequent blood transfusion.

Expression of Kidd antigens

The expression of the Kidd antigens is limited to RBCs and the kidney (in the vasa recta).

Function of Kidd protein

The Kidd protein is a major urea transporter in RBCs. It rapidly transports urea into and out of RBCs and in the process helps to maintain osmotic stability. The urea transport across Kidd null RBC membranes is

1000 times slower than across normal RBC membranes (2, 3).

The transport of urea by the Kidd glycoprotein in the kidney enables the kidney medulla to maintain a high concentration of urea, which in turn enables the kidney to produce concentrated urine.

However, the absence of the Kidd glycoprotein is not associated with disease. The RBCs in Kidd null individuals have a normal shape and lifespan (3). Individuals with the Jk(a-b-) phenotype are unable to maximally concentrate urine, but it does not cause any other health problems (4).

Clinical significance of Kidd antibodies

The Kidd antibodies are often difficult to detect, making them hazardous in transfusion medicine, where they are suspected to be a common cause of delayed hemolytic transfusion reactions (DHTRs) (5).

Transfusion reactions

Anti-Jk a can cause severe and fatal hemolytic transfusion reactions (6) but is more commonly associated with less severe DHTRs. It has been estimated that over one-third of DHTRs are caused by anti-Jk a (7, 8). Case studies have also pointed to anti-Jk b as being responsible for severe DHTR (9, 10). Anti-Jk3 has also been responsible for causing severe hemolytic transfusion reactions, both immediate and delayed (5).

Hemolytic disease of the newborn

During pregnancy, fetal Kidd antigens are capable of causing alloimmunization of the mother (11). But in contrast to the hemolytic activity of Kidd antibodies in incompatible blood transfusions, anti-Jk a and anti-Jk b are only rarely responsible for severe HDN (12). Likewise, anti-Jk3 is a rare cause of HDN, but the first documented case in Mrs. Kidd’s newborn was fatal.

Molecular information

The SLC14A1 gene (Solute carrier family 14, member 1) is a member of the urea-transporter gene family and is located on chromosome 18 (18q12-q21). The gene is organized in 11 exons distributed across than 30kb of DNA. The first three exons and part of the fourth are not translated; exons 4-11 encode the mature Kidd protein.

The Jk a and Jk b antigens are the products of two alleles that are inherited in a co-dominant fashion. The Jk a /Jk b polymorphism results from a 838G→A transition, resulting in an D280N substitution (13). Based on this, several investigators have suggested different methods for JK genotyping (13–15).

The Jk(a-b-) phenotype is generally inherited as a recessive trait—a number of different mutations have been found to be responsible 16). In the Polynesian population where the null phenotype is less rare, a splice site mutation causes loss of exon 6 from mRNA transcripts and it is unlikely that the truncated Kidd protein produced is transported to the RBC membrane (17). A similar situation holds true in the Finnish population in which another genetic explanation causes the same phenotype (17, 18).

View the sequences of Kidd alleles at the

The Kidd protein urea transporter is an integral protein of the RBC membrane. It is a transmembrane protein containing 389 amino acid residues. The protein is predicted to span the membrane 10 times with both the N terminus and C terminus being intracellular. This membrane topology is shared by the anion exchanger that bears the Diego blood group antigens.

The Kidd protein consists of two hydrophobic domains that each span the membrane five times, and they are linked by a large glycosylated extracellular loop. The Asn211 on this third loop carries 1% of ABO antigens found on the RBC. The Jk a /Jk b polymorphism is found on the neighboring fourth extracellular loop (19).

Antigen & antibody overview

Red cell antibodies are produced in response to an

Safe and effective red cell transfusions are based on your knowledge of red cell antigens and antibodies.

This knowledge also forms your understanding about the aetiology of haemolytic disease of the newborn and its clinical management.

These are either carbohydrate or protein substances that trigger our immune system to produce antibodies.

Antigens are found on the surface of red cells, white cells and platelets but are also found on body tissue, body fluids, and on surfaces of bacteria and viruses.

Antigenic cellular blood elements and plasma proteins can result in alloimmunisation or the production of antibodies, specifically called alloantibodies, which are directed against the blood group antigens of another individual.

The immune system’s response to some of these antigens range from clinically benign to massive and complicated immune reactions, possibly resulting in death.


These substances are produced by our body’s immune system in response to an antigenic stimulus.

Antibodies or immunoglobulins (Ig) are made of gammaglobulins, consisting of five main types: IgG, IgM, IgA, IgD and IgE. Each type performs a different role. Almost all antibodies to red cells are IgG or IgM, and a minority have an IgA component.

IgM antibodies are naturally occuring and may result from ‘natural’ exposure to substances within the environment or diet that have a similar structure to the corresponding red cell antigens. They are generally non-reactive at 37 °C and are therefore of little clinical significance, however IgM anti-A and anti-B are potent haemolysins and are capable of causing intravascular haemolysis by binding complement and are therefore classified as clinically significant antibodies.

IgG antibodies are formed as a result of immunisation following exposure to foreign red cell antigens through transfusion of red cells or by fetal red cells leaking into maternal circulation during pregnancy.

These IgG antibodies are reactive at 37 °C and are clinically significant because they are capable of mediating destruction or sequestration of transfused allogeneic red cells. They are known to cause haemolytic transfusion reactions or haemolytic disease of the fetus and newborn.

Rh Negative Blood and Pregnancy

As soon as your pregnancy is confirmed your midwife or doctor will recommend blood tests to determine your immunity to various illnesses, as well as your overall health, your blood type and your Rh factor. Not everybody knows what their blood type is as it generally doesn’t affect us in a day-to-day sense. However in pregnancy it becomes important, as does the knowledge of whether you are Rh positive or negative.

What do Rh negative and Rh positive mean?

Everyone has a blood type (O, A, B or AB) and also a positive or negative rhesus or Rh factor. If you are Rh negative (also referred to as Rh (D) negative), this means your red blood cells do not have the D antigen. The D antigen is a protein carried on the red blood cells of people who have Rh positive blood types. Seventeen percent of the population are Rh negative and this is also influenced by ethnicity.

What is the significance of Rh negative blood and pregnancy?

A foetus inherits its blood type from both its mother and its father, therefore your baby may not have your blood type. An Rh negative mother for instance may carry an Rh positive baby.

Normally this isn’t an issue unless the foetal blood enters the mother’s blood stream. If this occurs then the mother’s body will develop antibodies to destroy Rh positive antigens because it recognises these as foreign cells. This is quite similar to the way the human body develops antibodies to fight viruses and provide ongoing immunity when a person comes into contact with a transient illness.

If the mother’s body develops antibodies to Rh positive blood then this can have catastrophic effects on future pregnancies as the antibodies cross the placenta and attack the foetus’ positive red blood cells. This can cause severe anaemia, fluid retention, swelling and brain damage in the foetus, and potentially foetal heart failure and death. This condition is called Haemolytic Disease of the Newborn or HDN.

How can foetal blood cross into the mother’s blood stream?

Foetal blood can cross into the mother’s blood stream without the mother being aware of it. The most likely time this will happen is during childbirth thus affecting future pregnancies. Times when foetal blood may cross into the mother’s blood stream include:

  • Childbirth (affecting future pregnancies)
  • Miscarriage beyond 12 weeks gestation
  • Ectopic pregnancy
  • Termination of pregnancy
  • Amniocentesis
  • Chorionic Villus Sampling (CVS)
  • Vaginal bleeding during pregnancy
  • External Cephalic Version (ECV)
  • An accident resulting in a hard blow to the stomach

Who is at risk?

Only mothers who have Rh negative blood are affected in pregnancy. Mothers who are Rh positive are not affected even if their baby is an Rh negative blood type. If an Rh negative mother is carrying an Rh negative baby then this will not affect her or her baby in this pregnancy. However we do not commonly know our baby’s blood types before birth to determine if the baby is Rh positive or Rh negative.

How can Rh negative mothers protect their pregnancies?

If you have an Rh negative blood type your caregiver will recommend you receive two injections during pregnancy, at 28 weeks and 34 weeks respectively, and a third injection following childbirth. These injections are Rh (D) immunoglobulin, commonly referred to as the anti D injection, which means they prevent the mother’s body creating antibodies to fight the positive blood group of this foetus or future foetuses. It is important to note that if the mother already has the Rh antibodies, the immunoglobulin will be ineffective as it is a preventative only.

The anti D injection is made of blood plasma and has been used since the 1960’s with no major adverse reactions known to affect either the mother or the foetus. In rare cases it may cause a mild allergic reaction in the mother such as a rash or flu-like symptoms. Therefore it is advisable to remain at the hospital or doctors clinic for 20 minutes following the first injection.

What happens if I develop Rh positive antibodies?

The development of antibodies against Rh positive blood is called rhesus disease and occurs in 16% of Rh negative women if not given the anti D injection. This is referred to as sensitisation which means the antibodies easily cross the placenta into the baby’s amniotic fluid and blood stream via the umbilical cord in future pregnancies.

If rhesus disease is present then the level of sensitisation will determine the level of monitoring or interventions needed. If levels of antibodies are low then your baby may not require any treatment. In more serious cases s/he may require phototherapy, a treatment used for jaundiced newborns. Your baby may also require blood transfusions in utero or after birth to speed up the removal of bilirubin in his/her blood.

Bilirubin is produced during the normal breakdown of red blood cells. When bilirubin is present in quantities above what the liver can excrete, it causes an orange tinge to the skin and eyes known as jaundice, which we sometimes see in newborns.

Need more information?

The Australian Red Cross Blood Service (2010) has produced an informative, easy to understand brochure for pregnant women who are Rh negative which can be found here. Your midwife or doctor will also be able to provide you with information regarding Rh negative blood types and the anti-D injection.

Article written for Pregnancy Birth and Beyond on 14 th October 2014


Australian Red Cross Blood Service 2010, You and your baby: important information for Rh (D) negative women, CSL Biotherapies, Australia.

Dean, L. 2005, Blood groups and red cell antigens, National Center for Biotechnology Information, United States.

National Health Service 2013, Rhesus disease, United Kingdom.

NSW Health 2014, Maternity – Rh (D) Immunoglobulin (Anti D), Sydney.

Blood Bank Ch.7 (antibody detection & ID)

Women who are pregnant or following delivery

Patients with suspected transfusion reactions

Blood & plasma donors

Must incubate patient’s serum with screening cells using IAT procedure* (this is required by FDA)

A reaction to one or both of the screen cells demonstrates the presence of an atypical ab

The most common clinically significant ab react with a two-cell or three-cell screen

The screen provides initial clues that begin the ab identification process

An antigram is provided for each panel of cells (panel cells of identification)

Panel antigram is unique to each panel lot number

Known antigens are indicated with "+" if present on cells & with "0" if absent

normally start with 11 panel cells plus an autocontrol

Patient’s plasma is combined with his or her own red cells (4% solution for tube method)

Alternatively it may be set up with the screen

Includes the potentiator used in the antibody screen

Read tubes; IS, 37C, IAT

If the autocontrol is negative, an alloantibody is indicated

If the autocontrol is positive, an autoantibody is suspected if not transfused recently

If the autocontrol is positive & the person has been transfused within the last 3 months, it is possible that an alloantibody is present in the recipient & is binding to the donor RBCs in circulation

If the autocontrol is positive the next step is a full DAT

False positives can be caused by:

Autocontrol (not required for routine testing)

Positive antibody screen

An incompatible crossmatch

A transfusion reaction

Positive DAT after transfusion

Clinically significant IgG ab which can cause transfusion reaction

Low tittered antibodies

Antibodies to low frequency antigens

Antibodies that do not react under experimental conditions used (pH, LISS, other ions, etc..)

Group O is used to avoid any interference from naturally occurring ABO antibodies

Includes homozygous Duffy & Kidd antigens

R1R1, R2R2, rr, cell configurations

Homozygous = sensitive assay

Each vial represents the red cells harvested from one single donor

AABB requires that all recipient specimens be tested with UNPOOLED cells (ensures max sensitivity)

Screening cells licensed by the FDA require the antigenic profile capable of detecting the most clinically significant antibodies

Cells must not be used past their expiration date or when hemolysis or discoloration is observed

All red cell reagents expire every 3 weeks*- BB labs have standing orders

D, C, c, E, e, M, N, S, s, LeA, LeB, P1, K, k, FyA, FyB, JkA, JkB

Increase sensitivity of test with potentiators; albumin (BSA), LISS, PEG, enzymes

Incubation time = 15 minutes, used b/c it is short

Can enhance reactions up to 2 hours as long as the tube does not dry out

Mechanism of action is unclear

Enhances sensitivity at AHG phase for most antibodies

Does not enhance warm autoantibodies

Limitation: requires increased incubation time

Important to have equal volumes of serum & LISS to provide the final low ionic strength required

Limitations: Enhances cold autoantibodies & may miss weak Kell antibodies

Limitation: It tends to make RBCs aggregate so it is difficult to determine accurate results

Solution: after addition of PEG do not centrifuge until saline has been added to dilute the PEG

Proteolytic = causes breakdown of protein molecules

Enzymes modify RBC membrane & denature certain antigens

Enhance reactions of Rh, Kidd, Lewis, I, P system antibodies

Denatures antigens of M, N, XgA, FyA & FyB

Always used as an additional test, NEVER as the only test

Enhanced sensitivity; a 1+ with tube gives you a 3+ with enhanced methods

Can catch low titer antibodies such as Jk

Can be automated

Pulls up non-clinically relevant antibodies which can take hours to resolve

More important for BB labs than any other

Built in systems/checks in place to prevent mistyping, missing antibodies & other clinically relevant information

Each patient file is pulled up by tech barcode (or unique identifiers) on the specimen

List of patient identifiers

ABO/Rh type, Ab screen results, race, type of eligibility for cross matching are in header

Tabs; antibodies, problems/complications, patient history, previous transfusions, comments

Patient history is accessed before working up test

Often someone else has performed the test before & this gives an indication of what to expect/confirm

Even if certain tests were performed previously you MUST repeat what is requested

Patient’s typing results need to be confirmed

Presence or lack of antibodies is confirmed

Making assumptions = screw ups, don’t assume anything

Alloantibodies following transfusion (may be seen as a mixed field)

Wash 1 drop of 4% patient cells 3-4 times with saline (90 sec centrifugation each wash)

Add 2 drops of polyspecific AHG (IgG, C3d)

Centrifuge on IS (45 sec)

Read & record results

Screen interpretation results have 3 columns; IS, 37C & AHG

If reaction only seen in AHG, can mean alloantibody or warm auto-ab (if all same strength)

If reactions seen in multiple columns, could mean multiple antibodies present

If reaction only seen in IS, could mean autoantibody

IgM reacts at room temp or on immediate spin (IS) = anti-Le, M, N, I, & P; cold reacting Ab’s

IgG reacts at 37C &/or at the AHG (antiglobulin) phase= anti-Rh, Kell, Kidd, Duffy, SsU, LuB; Warm reacting Ab’s

Reactions at different phases may suggest the presence of both IgG & IgM classes (anti-M often contains a mixture of both IgM & IgG classes of antibodies)

Varying strengths may suggest multiple antibodies or a single antibody showing dosage

Antibodies to K, D, E, e, c, C agglutinate strong even with heterozygous cells

Antibodies to FyA, FyB, JkA, JkB, S, s agglutinate weakly, especially with heterozygous cells

The phase of reaction(s) indicates whether antibody is IgG, IgM or if both may be present

Reaction strength is a clue to the number of antibodies present

Heterozygous cells (in some cases) can show weak reactions

Rule out negative cells

Negative heterozygous cells should NOT be ruled out because the antibody may be too weak to react at all, Exceptions to this are K+k+, E & C

Match the pattern, look at reaction that are positive

Use rule of three

Phases: RT, 37C, AHG

Reaction strength: same or variable

Ruling out: use negative reactions

Match the pattern

Rule of three: 3 pos + 3 neg

Phenotype patient: pos or neg for antigen

At least 3 cells lacking the antibody specificity you suspect do not react (panel cells do not possess the antigen for the antibody)

If not enough cells in the panel meet these requirements cells from additional panels may be selected

Why 3 cells: Statistical, with 3 positives & 3 negatives this gives you a 95% chance that you correctly identified the antibody

Patient cells should be tested to ensure that they are negative for the corresponding identified antibody

Red cells from recent transfusions may cause misleading or incorrect results

Tested by antigen typing with known antisera

Enzymes can be used to eliminate or enhance antibody activity

Reactions may vary in multiple phases

Reactions may vary in strength

Selected cells from other panels may be used

All reactions occur in the same phase

Reaction strengths are the same

Samples may need to be sent to a reference lab

DTT: reagent that reduces disulfide bonds (destroys some epitopes)

Re-testing the patient’s serum with DTT treated cells may help identify Kell antibodies or eliminate interference (k)

Although not used in routine testing in transfusion services, it is used in reference labs

Can be caused by cold autoantibody, warm autoantibody, delayed transfusion reaction

Initial investigation should include a DAT*

Cord cells can help with cold antibody identification

Warm autoantibodies have varying significance

Elution & adsorption techniques ensure the absence of underlying antibodies

Patient diagnosis, transfusion history, medications can be consulted

PI, M, N, LeA, LeB are examples

These are usually clinically insignificant & won’t cause RBC destruction if antigen positive, incompatible units are transfused

However, ideally when cross match is performed, the antibody activity should be avoided to find compatible blood

Cold alloantibodies detected at IS may not continue to AHG**

Neutralization or pre-warming techniques are useful

Example: use Lewis substance to neutralize anti-LeB

Commercial neutralizing substances available for A, B, H, PI, Le

Panel cells: positive at IS

DAT: positive with anti-C3 (IgM; complement)

Other clues: ABO discrepancy may be first clue

If one knows there is any cold antibody present, practically it does not matter the exact specificity- you just want to avoid it so you can test for clinically relevant IgGs

Short cold panel: select cells that will help identify the suspected cold autoantibody

Always include a cord cell since it is I, Le & P negative

Select screening cells I, II, III, auto, cord

A1 & A2 cells may be included, but only if patient is group A or AB

Most common cold autoantibodies are I, IH, H

Group O individual:

Anti-I reacts with all cells that have the I antigen: (SC I, SC II, SC III, auto, cord)

Anti-I is negative with all cells that have i antigen (Group O cord cells)

Group A individual:

If anti-I, plasma reacts with all cells that have I antigen (adult)

Anti-IH: Because group A cells have so little H they can produce a cold auto against H. More commonly it is anti-IH. Therefore, compared to the autocontrol, cord RBCs normally react weakly (little I) & A2 cells react stronger than A1 cells (A2 have more H antigen)

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