Antibody Detection and Identification

The procedure for antibody detection and identification begins with patient serum or plasma mixed with reagent red cells. If a measurable amount of antibody is present in the patient's plasma or serum, and the reagent red cells carry the corresponding antigen, this interaction leads to sensitization and crosslinking, ultimately causing a visible clumping pattern defined as agglutination. Agglutination or hemolysis indicates sensitization of the reagent red cells by an unexpected antibody in the patient's serum.

The manufactured reagent red cells are packaged with an insert known as an antigram or antigen profile sheet. Each reagent vial represents a single donor that carries a specific set of inherited antigens; the antigram shows this phenotype.

Antibody detection is performed using an antibody screening test. An antibody screen consists of 2 or 3 group O reagent red cells with known antigen phenotypes. A positive antibody screen signifies that an unexpected antibody is present in the patient's serum.

If the antibody screen is negative, no further studies are necessary. However, a positive result requires additional testing, such as identification with an antibody panel. If the antibody screen is positive, the antibody must be identified by performing an antibody panel. A panel often consists of 10 to 20 group O reagent red cells with known phenotypes.

Table 1 below depicts an antibody screening antigram. A "+" indicates the antigen is present on the cell and a "0" indicates the antigen is absent. The antigram will also show if a cell is homozygous or heterozygous for an antigen. For example, looking at the Kidd blood group system, cell 1 is heterozygous (Jk^a+ Jk^b+) while cell 2 is homozygous (Jk^a+ Jk^b-).