Weak D
anti-C3d and anti-IgI
polyspecific AHG reagent
direct antiglobulin test (DAT)
positive DAT indicates coating of RBCs with an antibody
partial D
some D-positive RBCs can be missing parts of the D antigen complex
IAT two-stage testing
stage 1: antibodies combine with antigens (requires incubation) stage 2: washing the RBC, followed by addition of AHG reagent
weak D position effect can be found when
the C allele is inherited trans to D
preventing HDFN by using
RhIg (passive IgG anti-D) - protects D negative mother from becoming immunized by exposure to D positive fetal cells
kernicterus
accumulation of lipid-rich tissue
if IAT is negative
add IgG-coated RBCs, agglutination is expected
positive IAT test
agglutination is observed after adding AHG indicated presence of D antigen on RBC
AHG
anti human globulin
(AHG) Anti-Human globulin are
antibodies against human antibodies
the D antigen
is second only to ABO antigens in important in transfusions
antigens and the placenta
- IgM cannot cross the placenta - IgG can cross the placenta
false negative IAT
- failure to add AHG - improper washing - under-centrifugation - loss of reagent reactivity (check cells) - under reading - weak red cell suspension
indirect antiglobulin test (IAT)
- used to detect in vitro sensitization of RBCs by IgG antibodies and/or complement - IgG like to react at body temperature so test is inducbated
false positive IAT
-positive DAT - dirty glassware - over-centrifugation - over reading
hemolytic disease of fetus & newborn (HDFN) erythroblastosis fetalis
-results from excessive destruction of fetal red cells by maternal antibodies - production of antibodies in the mother that have been stimulated by foreign (fetal) antigens
AHG is used to detect
IgG coating RBCs in humans
D antigen inheritance
chromosome 1
IgG-coated RBC (Coombs check cells)
control for negative IAT tests, require by the AABB
detection of weak D antigen is done with an
indirect antiglobulin test (IAT) - cannot be detected on an individual with positive DAT
negative IAT test
no agglutination observed after adding AHG, agglutination observed with IgG-coated RBC
immune-mediated antibodies (usually IgG)
- red-cell mediated (pregnancy, transfusion)
HDFN patho and clinical presentation
- anemia and jaundice (can be fatal, hydrops fetalis) - excessive bilirubin levels in neonate (can cause mental retardation, kernicterus)
administration of RhIg
- at 28 weeks gestation - after delivery of a D positive infant (within 72 hours) - after aminocentesis, miscarriage, or ectopic pregnancy (within 72 hours)
how AHG is made
- commercially prepared reagents - inject animal with human antibodies (globulins) - animal sees human antibody as foriegn and mounts an immune response - animal produces AHG - AHG is extracted from animal and purified
different forms of weak D antigen
- inherited weak D gene - position effect: C trans to D - partial D
testing donors for weak D
- the AABB requires testing for weak D on all donors that do not directly agglutinate with anti-D reagents - weak D positive units are labeled as D positive and should be transfused only to D-positive recipients - testing for weak D in recipients is only required in pregnant women - if D antigen is not detected, Rh (d) negative blood is provided