In general, these antibodies are stimulated following exposure to foreign antigens via transfusion or pregnancy, are IgG in nature, and do not fix complement. As IgG antibodies they react best at 37o C or following antiglobulin testing. They have the capability of causing transfusion adverse events (transfusion reaction, extravascular hemolysis) and, once documented, must continue to be recognized despite a drop in titer below detectable levels. As IgG molecules can cross the placenta, these antibodies must be followed during pregnancy using antibody titration procedures in order to provide useful, timely information regarding the potential to cause hemolytic disease of the fetus and newborn (HDFN).
The antibodies may show dosage effect*, demonstrating stronger reactions with reagent red cells that possess a double dose (homozygous genetic expression) of the antigen to which they are directed versus reagent red cells from donors with a single dose of the given antigen. In antibody identification workups, it may be advantageous to pretreat reagent red cells with enzyme in order to enhance reactivity of the antibody with the antigen on the pretreated cell.
*Example of dosage: Anti-E may exhibit a 3+ reaction with cells that are E+e- and a 2+ reaction with cells that are E+e+
