H e m a t o lo g y E d u c a t io n - European Hematology Association

S.R. Sloan
Harvard Medical School
& Children’s Hospital, Boston,
Mass., USA
Hematology Education:
the education program for the
annual congress of the European
Hematology Association
2011;5:381-384
Transfusion
Immunologic responders to red blood cell transfusions
Risks associated with anti-RBC
antibodies
Allogeneic red blood cell (RBC) transfusions
expose patients to dozens to hundreds of foreign
antigens that are exposed on erythrocyte
membranes. Remarkably, RBC transfusions
do not usually result in a detectable immune
response and transfused RBCs usually have a
normal lifespan. Occasionally, however, an
antibody response to one or more antigens
occurs. Such an immune response can be dangerous
for the patient, risking delayed
hemolytic transfusion reactions, and/or
hemolytic disease of the fetus and newborn.
The severity of a delayed hemolytic transfusion
reaction depends in large part on
whether the reaction is a primary immune
response or whether it is a secondary
immune response. If the response is a primary
immune response, then the patient can
develop a relatively mild delayed hemolytic
transfusion reaction. However, secondary
amanestic responses can be relatively rapid,
and strong responses can cause severe
delayed hemolytic transfusion reactions that
in rare cases, can be fatal.
Approaches to prevent risks associated
anti-RBC antibodies
Blood banks have procedures to avoid
severe delayed hemolytic transfusion reactions
associated with secondary immune
responses. These procedures are generally
aimed at detecting and identifying RBC
alloantibodies prior to RBC transfusions and
avoiding transfusing RBC units expressing
the corresponding antigens.
Although current blood bank approaches
prevent the most potential serious delayed
A B S T R A C T
Alloimmunization to RBC antigens is a significant risk factor for future transfusions and pregnancies.
Analysis of patient databases strongly suggests that there is a distinct group of potential
immunologic responders to RBC transfusions. This group, comprising 13–20% of the general patient
population, is most likely genetically determined and is not related to disease. However, even potential
immunologic responders do not respond to most transfusions. Though it is unclear what factors determine
which RBC transfusions will induce an immune response, animal models suggest that inflammatory
signals may contribute to responsiveness to individual transfusions. When a patient responds to
one antibody, he or she is more likely to make additional antibodies, but development of those additional
antibodies is a stochastic process. Youngest patients have the broadest repertoire of antibody
responses. The anti-RBC antibody repertoire gradually declines with age.
hemolytic transfusion reactions, they do not
prevent all such reactions. One deficiency in
the current system concerns the fact that
antibody screens are usually only performed
prior to the anticipated transfusions, not in
the weeks following RBC transfusions. Tests
performed several weeks after a transfusion
would be the most sensitive in detecting
transfusion induced antibodies. 1 Instead,
however, standard practice usually involves
waiting until a transfusion itns anticipated to
perform an antibody screen. Unfortunately,
this may not detect prior immune responses
because anti-RBC antibody titers often wane
over time, often to undetectable levels after
several years. 2 Although antibodies can
wane over time, memory lymphocytes can
persist throughout an individual’s life. These
memory lymphocytes can mount a rapid,
vigorous response, resulting in a severe
delayed hemolytic transfusion reaction even
when no detectable antibody was present
prior to a transfusion.
In addition to failing to prevent severe
delayed hemolytic transfusion reactions,
protocols primarily aimed at avoiding secondary
immune responses to transfused erythrocytes
ignore the risks posed by some
antibodies to women and girls with child
bearing potential. Some IgG antibodies can
be transported across the placenta and react
with fetal erythrocytes, resulting in hemolysis.
The resulting clinical syndrome,
hemolytic disease of the fetus and newborn,
can be severe but rarely occurs during the
first pregnancy of a non-transfused woman.
Recognizing the risks posed by primary
immune responses to erythrocyte antigens,
most modern transfusion services have
adopted protocols to reduce the incidence of
primary immune responses. Most transfusion
services have minimized transfusion of
Hematology Education: the education programme for the annual congress of the European Hematology Association | 2011; 5(1) | 381 |