RBC Group Systems, HLAs, & Platelet Antigens

Why are antibodies to low frequency antigens not commonly encountered?

antigen exposure produces antibody and most donated blood do NOT possess these antigens

Why are antibodies to high frequency antigens not commonly encountered?

Because most blood contains these antigens

(you cannot possess an antigen and its corresponding antibody simultaneously)

What are the two major antigens in the Kell blood group?

• K (K1 or Kell)

  • low frequency (very little people possess)

  • very clinically significant for HDFN

• k (k2 or cellano)

  • high frequency, over 90% of the population has this antigen

Kp^a and Kp^b

• Kell antigen alleles

• Kp^a is low frequency and Kp^b is high frequency

Js^a and Js^b

• Kell antigen alleles

• Js^a is low frequency (20% of black population, 0.1% white population)

• Js^b is high frequency (80% black and 100% white)

Kell antibodies

• IgG

  • clinically significant (antigen negative units must be given)

• RBC stimulated — by pregnancy or transfusion

• associated with HDFN and HTRs

• agglutination is best observed during the IAT

  • because it is IgG

• doesn’t bind complement

  • hemolysis is extravascular

• anti-K is the most common antibody in the Kell system

In the Kx blood group system, the XK gene codes for:

XK protein, which carries the KX antigen

• the KX system is phenotypically related to Kell

• the absence of the KX antigen weakens the expression of Kell antigens

What makes the KX blood group system different from other systems?

• the XK gene is sex-linked (it is found on the x chromosome)

When the KX antigen is not expressed on red cells (rare), an abnormality called McLeod phenotype occurs. What is this?

• causes McLeod Syndrome

• mostly seen in men

• results in acanthocytosis, decreased RBC survival, neurologic effects, and chronic granulomatous disease (CGD)

Duffy antigens

• Fy^a and Fy^b

  • codominant alleles

• white populations: Fy(a+b+), Fy(a-b+), and Fy(a+b-)

• black populations: Fy(a-b-)

  • certain malarial parasites cannot invade Fy(a-b-) red cells because Fy^a and Fy^b are used as receptors to enter

Duffy antibodies (anti-Fy^a and anti-Fy^b)

• IgG

  • best detected at IAT

• antigen negative units MUST be given

• RBC stimulated by pregnancy or transfusion

• associated with HTRs, not so much HDFN

• extravascular hemolysis (does not bind complement)

Kidd blood group antigens

• Jk^a, Jk^b, and Jk³

  • Jk³ is only present when Jk^a and Jk^b are present

• about half of black americans are Jk(a+b-)

• about half of white americans are Jk(a+b+)

• Jk(a-b-) null phenotype

  • common in East Asia and Pacific islands

Kidd antibodies (anti-Jk^a and anti-Jk^b)

• IgG

  • agglutination best at IAT

• clinically significant

• capable of causing HTRs and HDFN

  • common cause of delayed HTRs

    • why? antibody titers decrease to undetectable levels (antigen negative units MUST still be given)

• may bind complement

  • hemolysis is usually extravascular, but can be intravascular (due to possibility of binding to complement)

Lutheran antigens

• 20 antigens exist, but Lu^a and Lu^b are most important

• Lu^a has a low incidence and Lu^b has a high incidence (over 90% of the population)

• Lu_null phenotype is rare (inherited recessively)

  • no Lutheran antigens on red cells

Lutheran antibodies (anti-Lu^a and anti-Lu^b)

• anti-Lu^a

  • can be naturally occurring (not always RBC-stimulated)

  • IgM or IgG (but reacts best at room temp)

  • shows mixed field agglutination

  • rare, mild HDFN, NO clinical significance in transfusions

• anti-Lu^b

  • rare, IgG

  • reacts best the the IAT

  • also shows mixed field reactions

  • clinically significant: causes HTRs and mild HDFN

mixed field agglutination

agglutination pattern where some red cells are agglutinated and others are not

What makes Lewis antigens different from other blood group antigens?

• Le^a and Le^b are NOT alleles (you’re either Le+ or Le-)

• antigens are produced by tissue cells and secreted into body fluids

  • glycolipids absorb them onto RBC membranes

• antigens are greatly reduced on RBCs during pregnancy

Le(a-b+) red cell phenotype is the result of inheriting what 3 genes?

Hh, Se (secretor), and Le genes

If you inherit an Le allele, where can you find Le^a antigens?

• in body secretions

  • Le(a+b-)

What happens if you inherit an Le, Se, and H gene together?

• H antigen structures will be converted to Le^b antigen, which RBCs will absorb onto their membranes

What happens if you inherit le/le alleles from each parent?

you will have no Lewis antigens on your red cells, Le(a-b-)

Lewis antibodies

• produced by Le(a-b-) individuals ONLY

• can be naturally occurring

• IgM

  • agglutination occurs at IS phase

• not associated with HDFN

• not usually clinically significant

  • anti-Le^a can bind complement and cause hemolysis in VITRO

• RBC negative units are not necessarily required for transfusion

I blood group system antigens

• one antigen: I

• newborns have i antigen, older children and adults have I antigen

  • the i antigen has a linear structure and converts to a branched structure (I) as a child grows older

I antibody

• cold-reacting IgM

• clinically significant only in colder temperatures

• typically an autoantibody (autoanti-I and autoanti-i)

  • incubating a patient’s plasma with screening cells at 4 degrees C will show an autoantibody

• allo-anti-I is rare because most people have the I antigen

auto-anti-I and anti-i disease associations

• auto-anti-I

  • associated with Mycoplasma pneumoniae (walking pneumonia) and cold hemagglutinin disease (a cold autoimmune hemolytic anemia)

  • antigen negative units are not always required (because blood is at body temperature)

• auto-anti-i

  • associated with infectious mononucleosis, cold hemagglutinin syndrome (sometimes), and lymphoproliferative disorders

P1PK and Globoside blood group antigens

• two Globoside antigens: P and PX2

• three P1PK antigens: P¹, P^K, and NOR

• P1 phenotype

  • red cells have P, P¹, and P^K antigens, most common, NO antibodies

• p phenotype

  • null, negative for P, P¹, and P^K, very rare

  • anti-PP1PK antibody

anti-P1 and anti-PP¹P^K (P1PK and Globoside group)

• anti-P1

  • found in P2 individuals

  • IgM

  • not stimulated by RBCs (naturally occurring)

• anti-PP¹P^K

  • found in null phenotypes

  • clinically significant, causes in vitro hemolysis

auto-anti-P (P1PK and Globoside systems)

• IgG biphasic hemolysin (requires 2 different temperature phases) that binds to P¹ or P² cells at low temps in the patient’s extremities

  • when cells are warmed to 37 C, complement activates and hemolysis occurs

  • may appear in kids after viral infection

• also associated with cold paroxysmal hemoglobinuria

In the MNS system, ___ and ___ are alleles, and ___ , ___ , and ___ are alleles.

• M and N

• S, s, and U

MNS blood group antigens

• M and N antigens

  • found on glycophorin A molecules (proteins that are weaved into RBC membranes)

• S, s, and U antigens

  • found on glycophorin B molecules

  • U antigen is only present when S or s is inherited

    • (S-s-) = no glycophorin B = no U antigen

MNS antibodies

• anti-M

  • IgM AND IgG, usually clinically insignificant

    • however, IF anti-M reacts at the AHG phase, antigen neg units must be given

  • rarely causes HDFN

  • pH of 6.5 favors optimal agglutination

• anti-N

  • rare, IgM, not usually significant

  • may be found in dialysis patients

• anti-S, anti-s, and anti-U

  • IgG, clinically significant

  • anti-U is rare, but found in S-s- individuals

  • requires an IAT for detection (anti-U)

HLA antibodies

• antibodies to Human Leukocyte Antigens

• produced as a result of transfusion, pregnancy, or transplant

• associated with platelet refractoriness

  • the lack of a desired response (PLT increase) to platelet transfusions

• also associated with febrile transfusion reactions

  • has decreased though due to leukocyte reduced blood products

Identifying HLAs are ___ part of routine testing in the blood bank, but do have applications, such as:

• not

• matching for organ and stem cell transplants

Genes that code for HLAs are found in the:

• Major Histocompatibility Complex (MHC)

• divided into 3 classes:

  • Class I: antigens on PLTs, leukocytes, and nucleated cells

  • Class II: antigens on macrophages, dendritic cells, and B cells

  • Class III: codes for complement and cytokines

How are HLAs inherited?

individuals inherit one haplotype from each parent (codominant haplotypes)

lymphocytotoxicity test for identifying HLA antibody

• cells with HLAs are mixed with patient plasma (that may contain anti-HLA), complement, and dye

• if antibody is present, the complement cascade activates and lyses the cells

  • cells that take up the dye and stain dark are positive (HLA antibody is present)

What must be done to ensure graft acceptance in recipients?

matching HLA cells/tissues with preexisting HLA antibodies in the recipient (to avoid rejection reactions)

Patients can develop HLA antibodies by:

• pregnancy (3+): 30-50%

• blood transfusions: 50%

• having a previous transplant (90% of patients develop HLA antibodies after graft rejection)

antibodies to platelet antigens can cause:

• neonatal alloimmune thrombocytopenia (NAIT)

  • destruction of newborn platelets by maternal alloantibodies that target antigens inherited by the father

  • platelets for the newborn must either be antigen-negative PLTs from a donor or washed maternal PLTs

• posttransfusion purpura (PTP)

  • destruction of platelets after transfusion, causes small pinpoint bruises

What is the mnemonic for remembering which antibodies show dosage?

Mary Lew Really Kids Duffy

• M = MNS

• L = Lewis

• R = Rh

• K = Kidd

• D = Duffy

What is the mnemonic for remembering which Ag-AB reactions are enhanced with enzymes?

Kidd Lewis has Ripples on his Pecs and Abs

• K = Kidd

• L = Lewis

• R = Rh

• P = P1

• A = ABO

What is the mnemonic for remembering which antigens are destroyed by enzymes (ficin or papain)?

Many New Soldiers are Destroyed in the Field by eXplosions

MNS = MNS

D and F = Duffy

X = Xg^a

What is the mnemonic for remembering which blood groups have IgM cold reacting antibodies?

LIMP

• L = Lewis and Lutheran A

• I = I

• M = MN (not S)

• P = P1

Which reagent destroys Kell system antigens?

DTT