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2338 Part XIII: Transfusion Medicine Chapter 136: Erythrocyte Antigens and Antibodies 2339

EFFECT OF ENZYMES AND OTHER the reading frame during RNA translation. The resulting protein is trun-
cated and has no transferase activity. Another variant O allele encodes a
CHEMICALS ON ERYTHROCYTE transferase identical to that of B except it has arginine instead of alanine
ANTIGENS at amino acid position 268, which blocks the enzyme activity. A com-
prehensive listing of blood group alleles is available at the following
Expression of an RBC antigen is determined by its exposure as a result websites: http://www.ncbi.nlm.nih.gov/gv/mhc/xslcgi.cgi?cmd=bgmut/
of its position on the cell surface and its biochemical structure. Expres- home and www.isbt-web.org.
sion can be modified with treatment of RBCs by enzymes and other
chemicals. These reagents are used to help identify complex mixtures of
antibodies and to help characterize antibody specificity when identity is GENE COMPLEXES
not readily apparent. Some blood group genes are complexes of several closely linked genes
Proteolytic enzymes, such as ficin, papain, bromelin, trypsin, and or loci that evolved through duplication of an ancestral gene. The anti-
α-chymotrypsin, cleave proteins from the erythrocyte membrane at gens they encode are inherited as a haplotype with no or few crossovers.
specific amino acids. Enzyme treatment of RBCs cleaves certain protein Blood group examples include the Rh system with genes RHD and
antigens and allows carbohydrate and other resistant protein antigens RHCE, and the MNS system with genes GYPA, GYPB, and GYPE.
to react more strongly with their antibody. The reactivity of antibodies RHD and RHCE show remarkable homology between them and
with antigens in ABO, I, P1PK, LE, RH, and JK systems is enhanced with RHAG, which encodes the RhAG. GYPA and GYPB probably arose
after enzyme treatment of the RBCs, whereas reactivity of antibodies to by duplication of an ancestral GYPA gene encoding the N antigen. The
37
M, N, Fy , Fy , and many minor antigens (Xg , Ch, Rg, JMH, In , Ge2, most common MNS complex is Ns, followed by Ms, MS, and NS.
a
b
a
b
Ge4, Pr, Tn, and some examples of Yt ) is reduced or eliminated. S and In both RH and MNS systems, other antigens arose by further
a
s are variably affected by enzyme treatment, and Kell and Scianna anti- nucleotide changes, deletions, or rearrangements within the gene com-
gens are relatively unaffected. 4–6 plex. Unequal pairing of GYPA and GYPB during meiosis, with sub-
Reagents that reduce disulfide bonds, such as 2-mercaptoethanol sequent recombination, resulted in several hybrids, such as GYP(A-B)
(2-ME), dithiothreitol (DTT), and 2-aminoethylisothiouronium bro- (called Lepore type, by analogy with a similar hemoglobin hybrid),
mide (AET), denature Kell blood group antigens but enhance Kx. which encodes a protein with the amino-terminal end of GPA but the
Reducing reagents also denature antigens in LW, SC, IN, JMH, and YT carboxyl-terminal end of GPB. Anti–Lepore-type hybrids, GYP(B-A)
systems and weaken antigens in LU, DO, CROM, KN, and RAPH sys- (amino-terminal end of GPB and carboxyl-terminal end of GPA), and
tems and the AnWj antigen. 4–6 other rearrangements (e.g., GYP[B-A-B] and GYP[A-B-A]) are known.
Acid treatment of RBCs (ethylenediaminetetraacetic acid [EDTA]/ Within the Rh complex, numerous hybrids of RH(D-CE-D) and RH(CE-
glycine/acid reagent), which is frequently used to remove IgG from D-CE) have been identified. Such hybrids can result in altered antigen
RBCs, can weaken or completely denature antigens in the KEL blood expression and new antigens. 4–6
group system. Chloroquine treatment of erythrocytes (also sometimes Kell and Lutheran proteins are single-gene products that carry
used to remove IgG from RBCs) at room temperature has little effect on multiple antigens. The most common alleles in humans are kKp Js K
11
b
b
most antigens. However, treatment for 30 minutes at 37°C can weaken and Lu Lu Lu Au . Antigens of lower prevalence (K, Kp /Kp , or Js , and
c
a
a
8
b
a
6
expression of many antigens, including Fy , Lu , Yt , JMH, and those in Lu , Lu9, Lu14, or Au ) arise from separate nucleotide changes.
a
b
b
b
a
the RH, DO, and KN systems.
GENETICS OF ERYTHROCYTE ANTIGENS SILENT ALLELES
Some blood group alleles are amorphs, or silent; that is, they do not
Protein antigens are direct gene products: The gene encodes a protein produce a recognizable antigen, although they may encode a product
that expresses one or more antigens. Carbohydrate antigens, made that is simply not detected with standard test methods. As discussed
by transferase action, are indirect gene products. Most blood group with regard to the ABO system, A and B genes produce transferases
genes are located on autosomes; only two, Xg and XK, are located that add GalNAc or Gal, respectively, to the same precursors, but O
on the X chromosome (see Table 136–1 for locations of genes on produces no active enzyme. AB individuals express both A and B anti-
chromosomes). gen, but AA and AO individuals express A, and BB and BO individu-
Most genes that encode blood groups have two or more alleles. als express B. Amorphic alleles are recognized only in a homozygous
Individuals who inherit two identical alleles are homozygous and state, and the result is a “null” phenotype. Null phenotypes exist in
make a double dose of a single gene product, whereas those who most blood group systems (see Table 136–1). Group O is the most
inherit two different alleles are heterozygous and make single dose common, followed by Fy(a–b–) and Le(a–b–) in Africans. Other null
of each of two gene products. Males are hemizygous for the genes phenotypes are rare.
located on their single X chromosome and make a single gene prod- The Fy(a–b–) phenotype is especially interesting. Fy(a–b–)
b
b
uct. In contrast, females produce a double dose of the Xg and XK Africans have Fy genes that express normal Fy glycoprotein on tissue
gene products, as X-chromosome inactivation does not involve Xg a cells but not on RBCs. A nucleotide change that disrupts the GATA-1
38
or Kx antigens. 36 binding site for RBC transcription is present in these individuals,
which helps explain why many Fy(a–b–) Africans do not make anti-Fy
b
ALLELES despite exposure to antigen-positive RBCs from transfusion.
Alleles encoding blood group antigens commonly arise from only a
single or a few nucleotide changes. For example, A and B alleles differ GENE FREQUENCIES
by only seven DNA base substitutions, which result in four amino acid Gene and phenotype frequencies vary widely with race and geograph-
substitutions in their respective transferases. The common O allele is ical boundaries. 6,11,16,39 This information is useful when estimating the
4–6
similar to A except for a single base deletion at nucleotide 261 that shifts availability of compatible blood and the probability of HDFN.
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