Antibodies occur predictably in the sera of all normal adults in association with the ABO antigens. Demonstration of these antibodies is necessary for definitive classification of an individual's ABO cell type. The individual's serum is therefore tested against reagent red cells containing known antigens (see Table 3). This is called reverse typing.
Table 3. Testing the Patient Serum with Known Red Cells (Reverse Typing).| ABO Blood Group | Patient Serum Tested with Known Reagent Cells |
| A Cells | B Cells |
A | 0 | 3–4+ |
B | 3–4+ | 0 |
O | 3–4+ | 3–4+ |
AB | 0 | 0 |
+ = agglutination (graded 1+ to 4+); 0 = no agglutination or hemolysis
