The tests done in the blood bank are serological, i.e., they involve the interaction of antibody (in serum) with antigens (on red cells) and result in hemagglutination. Hemagglutination will result if serum contains an antibody specific for a red cell antigen, and the two are brought together under the optimal test conditions.
Hemagglutination tests done in test tubes are graded (from 4+ to 1+). They are read macroscopically, often with the aid of a lighted magnifying mirror. Some laboratories also read negative tests (especially antiglobulin tests) microscopically.
In blood banking, antibodies to red cell antigens may be IgM, IgG, or IgA. IgA antibodies are rare, and if present, usually occur with either IgM or IgG. Sometimes red cell antibodies with a single specificity are just IgM, sometimes they are just IgG, and sometimes they are both IgM and IgG.