Antibody Identification - Clinical Tree

Antibodies to RBC antigens except anti-A and anti-B are called unexpected antibodies. Unexpected RBC antibodies may be alloantibodies reacting against foreign antigens or autoantibodies reacting against self-antigens. Depending on the patient population, 1%–35% of patients have unexpected alloantibodies.

Once an alloantibody is detected on antibody screen, antibody specificity must be determined. Some unexpected alloantibodies may be clinically significant with potential to cause shortened survival of transfused RBCs, acute or delayed hemolytic transfusion reactions (HTRs), and hemolytic disease of the fetus or newborn (HDFN). Patients with clinically significant alloantibodies should receive RBCs that are negative for corresponding antigen.

Clinical Significance

RBC antibodies are termed clinically significant if they can cause hemolysis of corresponding antigen-positive RBCs resulting in HTR or HDFN.

Clinical significance of a specific unexpected RBC alloantibody is unknown but is presumed based on published reports. Alloantibodies may simply bind to transfused RBCs detected serologically, may lead to shortened RBC survival, or cause mild to severe acute or delayed HTR. Detection of alloantibodies and determination of their specificity (identification) is, therefore, important ( Tables 22.1 and 22.2 ). RBC autoantibodies are generally reactive against reagent and autologous RBCs. Transfused RBCs are generally expected to survive as well as patient’s own RBCs. Autoantibodies can cause difficulties in detection and identification of underlying clinically significant alloantibodies.

Table 22.1

Summary of Major Blood Group Antigens

Modified from Hillyer, C. D., Stauss, R.G., & Luban, N. L. C. (Eds.). (2004). Handbook of pediatric transfusion medicine . San Diego, CA: Elsevier Academic Press.

Antigens	Systems	IgM	IgG	Transfusion Reactions	Hemolytic Disease of the Fetus or Newborn (HDFN)	Prevalence (%)
Antigens	Systems	IgM	IgG	Transfusion Reactions	Hemolytic Disease of the Fetus or Newborn (HDFN)	White	African-American
A	ABO	X	X	Mild–severe	None–moderate	40	27
B	ABO	X	X	Mild–severe	None–moderate	11	20
D	Rh	X	X	Mild–severe	Mild–severe	85	92
C	Rh		X	Mild–severe	Mild	68	27
E	Rh	X	X	Mild–moderate	Mild	29	22
c	Rh		X	Mild–severe	Mild–severe	80	96
e	Rh		X	Mild–moderate	Rare	98	98
K	Kell	X	X	Mild–severe	Mild–severe	9	2
k	Kell		X	Mild–moderate	Mild–severe	99.8	>99
Kp ^a	Kell		X	Mild–moderate	Mild–moderate	2	<1
Kp ^b	Kell		X	None–moderate	Mild–moderate	>99	>99
Ja ^a	Kell		X	None–moderate	Mild–moderate	<1	20
Jp ^b	Kell		X	Mild–moderate	Mild–moderate	<99	99
Fy ^a	Duffy		X	Mild–severe	Mild–severe	66	10
Fy ^b	Duffy		X	Mild–severe	Mild	83	23
Jk ^a	Kidd		X	None–severe	Mild–moderate	77	92
Jk ^b	Kidd		X	None–severe	None–mild	74	49
M	MNS	X	X	None	None–mild	78	74
N	MNS	X		None	None	70	75
S	MNS		X	None–moderate	None–severe	52	31
s	MNS		X	None–mild	None–severe	89	94
U	MNS		X	Mild–severe	Mild–severe	100	>99
Le ^a	Lewis	X		Few	None	22	23
Le ^b	Lewis	X		None	None	72	55
Lu ^a	Lutheran	X	X	None	None–mild	8	5
Lu ^b	Lutheran	X	X	Mild–moderate	Mild	>99	>99
Do ^a	Dombrock		X	Rare	+DAT/No HDFN	67	55
Do ^b	Dombrock		X	Rare	None	82	89
Co ^a	Colton		X	None–moderate	Mild–severe	>99.9	>99
Co ^b	Colton		X	None–moderate	Mild	10	10
P ₁	P	X		Rare	None	79	94

Table 22.2

Clinical Significance of Antibodies to the Major Blood Group Antigens

Reproduced from Hillyer, C. D., Stauss, R.G., & Luban, N. L. C. (Eds.). (2004). Handbook of pediatric transfusion medicine . San Diego, CA: Elsevier Academic Press.

Usually Clinically Significant	Sometimes Clinically Significant	Clinically Insignificant If Not Reactive at 37°C	Generally Clinically Insignificant
A and B	At ^a	A ₁	Bg
Diego	Colton	H	Chido/Rogers
Duffy	Cromer	Le ^a	Cost
H in O _h	Dombrock	Lutheran	JMH
Kell	Gerbich	M, N	Knops
Kidd	Indian	P ₁	Le ^b
P, PP ₁ P ^k	Jr ^a	Sd ^a	Xg ^a
Rh	Lan
S, s, U	LW
Vel	Scianna
Vel	Yt

Unexpected Antibody Identification

Unexpected RBC antibodies can be detected initially as ABO typing discrepancy, positive antibody screen, incompatible crossmatch, or through eluate. Once detected, unexpected antibody(ies) must be identified to determine their clinical significance and requirement for antigen-negative RBCs.

Preanalytical Considerations

Age

Infants who are <6 months old usually do not produce antibodies, and identified antibodies are generally maternal due to placental transfer. Maternal plasma is used for crossmatching.

Gender

Women are more often sensitized to RBC antigens due to sensitization from paternally inherited antigens on fetal RBCs. Pregnancy history may be important in antibody identification.

Transfusion History

Prior transfusion of RBC-containing component is a possible sensitizing event. Timing of most recent transfusion is critical, as transfused RBCs can circulate for up to 3 months potentially complicating serologic testing results and interpretation.

Ethnicity

Certain antigens may be of high or low frequency in specific populations. Knowledge of the patient’s ethnic origin may provide clues to antibody specificity.

Medical History

Certain diseases have been associated with RBC antibodies. Cold agglutinin syndrome, Raynaud phenomenon, and infections with Mycoplasma pneumoniae are often associated with anti-I autoantibodies. Infectious mononucleosis is associated with anti-i autoantibodies. Patients with paroxysmal cold hemoglobinuria may demonstrate autoantibodies with anti-P specificity. Systemic lupus erythematosus, multiple myeloma, chronic lymphocytic leukemia, and lymphoma are associated with warm autoantibodies. Patients who have received solid organ or hematopoietic stem cell transplants may demonstrate passive antibodies that originate from donor passenger lymphocytes.

Drugs

Drugs are known to cause antibody identification problems. Other sources of drug-related problems during antibody testing include recent administration of intravenous immune globulin and Rh immune globulin.

Specimen Requirements

Serum or plasma can be used for antibody testing; however, EDTA-anticoagulated specimens are preferred due to potential for in vitro uptake of complement components by RBCs in clotted specimens.

Reagents

Antibody Identification Panel

Antibody identification requires testing the patient’s plasma against panel of selected RBC samples (typically 8–14 reagent RBCs) with known antigen expression of major blood groups (Rh, Kell, Kidd, Duffy, and MNS) ( Fig. 22.1 ). Reagent RBCs obtained from commercial vendors are supplied with accompanying panel with antigen expression profile of each reagent RBC. Panel RBCs are generally group O, thereby allowing plasma of any ABO group to be tested. Reagent RBCs are selected so that if one takes all the examples of RBCs into account, a distinctive pattern of positive and negative reactions exists for each of the many antigens (including D, C, E, c, e, M, N, S, s, P1, Le ^a , Le ^b , K, k, Fy ^a , Fy ^b , Jk ^a , and Jk ^b ). Selected RBCs in a panel should allow for identification of single specificities of common alloantibodies with exclusion of most others.

In patients with known alloantibody history, selected cell panel of corresponding antigen-negative RBCs is a better approach to identify any newly formed antibodies. It is not necessary to reconfirm the previously identified antibody(ies). If the patient’s phenotype is known, a selected panel may be tested to demonstrate the presence or absence of possible alloantibodies minimizing amount of testing required.

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