Classical erythrocyte immunohematology is based on the detection of reaction between the antigens present on the sur face of the red blood cells and the antibodies present in the antisera (or in patient serum). The antigen–antibody reaction can produce a variety of observable results. In blood group serology, the most observed reactions are agglutination, hemolysis, and precipitation.
Agglutination is the most widely used antigen–antibody reaction in erythrocyte immunohematology and consists of the aggregation, mediated by antibodies, of red blood cells that have the relative antigen on their surface. Agglutination of red blood cells occurs because the antibody molecules bind to the antigenic determinants of different but adjacent red blood cells, binding them together to form a visible aggregate.
Hemolysis consists of the rupture of red blood cells, resulting in the release of intracellular hemoglobin. In vitro hemolysis is a two-stage reaction: In the first stage, there is the binding of antibody to the blood group antigens; and in the second stage, there is activation of complement system leading to RBC destruction. Hemolysis is considered a positive result, because it demonstrates the antigen–antibody reaction with activation of the complementary cascade.
Precipitation consists of the formation of an insoluble complex, usually visible, which originates from the reaction of a soluble antibody with a soluble antigen. These insoluble antigen–antibody complexes are detectable in tube tests as an annular or button-shaped sediment or in agar tests as a precipitation line (Fig. 1).
Fig1. Agglutination, hemolysis and precipitation
