·
"B is Bad,
but A is Awful." The A antigen is more antigenic than B, and
anti-A antibodies tend to cause more severe hemolysis.
·
Infants with pyruvate
kinase deficiency have anemia at birth, invariably develop prolonged
hyperbilirubinemia, and will typically have a very
high nucleated RBC count. The latter is pathognomonic since mature RBCs
have almost no enzyme and cannot survive, and it is the normal RBCs that
perform most RBC functions. Similar to G6PD deficiency, the erythroid marrow is
markedly hypertrophied.
·
Some silent carriers of a-thalassemia produce
small quantities of an unusual hemoglobin called Constant
Spring. This hemoglobin was named after a small Jamaican town in
which the first family known to carry the gene resided.
·
In normal RBCs, a small amount of MetHb is
constantly being formed, but the level remains low (<1% of total Hb) because
of the presence of two enzyme systems, the NADH-dependent
cytochrome b5 MetHb reductase system and NADH
diaphorase. Cytochrome b5 reductase uses NADH to reduce cytochrome
b5, which in turn reduces MetHb to Hb. A small amount of MetHb is reduced to
hemoglobin by NADPH diaphorase. The
latter mechanism is enhanced by methylene blue, the
standard treatment for methemoglobinemia. Cellular antioxidants such as vitamin
C and glutathione may reduce MetHb to Hb nonenzymatically.
During the period up to 6 months of age, there is a
relative deficiency of NADPH diaphorase. This deficiency not only
predisposes persons to methemoglobinemia but also leads to a lack of response to methylene blue. Fetal
hemoglobin is more sensitive to oxidation than adult hemoglobin.
·
Anemia of
prematurity is not responsive to nutritional support with iron, folate, or
vitamin E. Transfusions may be necessary if the infant is ill or has
cardiovascular manifestations of anemia. EPO has been used with mixed results
in this condition.
·
Neonatal
alloimmune thrombocytopenia (NAIT): Just as RBCs have surface
antigens that make it possible to classify persons into "blood
groups," platelets have surface antigens as well. These are now called
human platelet antigens (HPAs). There are over 10 different HPAs. NAIT is a platelet incompatibility between mother and
fetus analogous to Rh disease. As with Rh disease, the mother makes
antibody against "paternal" antigens (most commonly HPA-1a,
previously called PLA-1). The antibody crosses the placenta and binds fetal
platelets, resulting in their destruction. The antibody can also cause a defect
in platelet aggregation.
·
The absence of
bleeding at routine circumcision does not rule out the diagnosis of severe
hemophilia.
·
Deficiencies of factor
XII, prekallikrein, and high-molecular-weight
kininogen. (contact factors)
they are critical to initiating the activation of coagulation in the in vitro
clotting assays but do not contribute significantly to
hemostasis in vivo. Although there is no true factor deficiency, it is
important to keep in mind that heparin contamination may result in a prolonged
PTT without bleeding.
Factor XIII deficiency is
a clinical bleeding disorder in which the screening tests, PT, PTT, and
platelet count are all normal.
·
Testing a newborn is
not recommended because the levels of the von Willebrand's factors may be
falsely high in the perinatal period as a result of the effect of maternal
hormones. For this reason, infants with type I do not present with
bleeding symptoms during this time. Testing is best performed after the age of
6 months and includes assays for factor VIII, von Willebrand's antigen, and the
ristocetin cofactor. Further testing of the multimer distribution is warranted
if type II is suspected.
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