The two principal subgroups of A are A1 and A2, although other subgroups exist. Serologic distinction between A1 and the other subgroups is based on agglutination of A1 cells with lectin anti-A1, prepared from Dolichos biflorus seeds. The red cells of approximately 80% of group A individuals are A1; the remaining 20% are A2 or weaker. Weaker subgroups of A or AB may produce anti-A1.

Serum containing anti-A1, whether it is naturally occurring or passively transfused, will agglutinate all A1 cells, but not A2 cells or any other subgroup of A. The serum of group A1 patients does not contain anti-A1 unless it has been passively transfused from a non-group A blood component, such as group O platelets.


The same type of sample that is used in regular ABO testing may be used.


  • 12 x 75 mm test tubes 
  • Reagent Lectin Anti-A1
  • Indelible marking pen 
  • Reagent A1 Cells
  • Plastic test tube rack 
  • Reagent A2 Cells
  • Dispo pipets Serofuge
  • Wash bottle with isotonic saline 
  • Lighted view lamp


  1. Prepare a washed 3% suspension of the patient's cells.
  2. Label 5 tubes: patient's initials lectin
  • lectin +
  • lectin neg.
  • patient last name A1C
  • patient last name A2C.
  1. Add one drop Reagent Lectin Anti-A1 to each lectin tube.
  2. Add 2 drops patient serum to the A1C and A2C tubes.
  3. Add one drop patient's 3% cell suspension to the patient lectin tube.
  4. Add one drop A1 cells to the lectin + tube and to the patient A1C tube.
  5. Add one drop A2 cells to the lectin neg tube and the patient A2C tube.
  6. Shake to mix and centrifuge the time appropriate for the saline spin calibration of the serofuge.
  7. Gently resuspend and examine macroscopically for agglutination using the lighted view lamp.
  8. Grade and record results on the worksheet.


  • Strong agglutination of the patient's cells with lectin anti-A1 indicates he or she is group A1 or A1B.
  • Failure to agglutinate in the lectin but agglutination with regular Anti-A indicates the cells are a weaker subgroup of A, probably A2.
  • Weak agglutination of the patient's cells with lectin anti-A1 indicates his or her cells have the A intermediate subgroup (Aint).
  • Agglutination of the patient's serum with A1 cells but not A2 cells indicates the presence of anti-A1 in the patient's serum. The source of this antibody may be either naturally occurring in weaker subgroups of A, or may be passively transfused from group 0 or B plasma products, usually platelets.
  • Group A or AB recipients of a transplanted group 0 organ may also have anti-A1 in their serum, produced by lymphocytes in the transplanted organ.


Regardless of the source, if the anti-A1 is reactive in the Coombs phase of the crossmatch, and it cannot be pre-warmed away, the patient should be transfused with group A2 or O packed cells. Consult the blood bank medical director for the appropriate choice of blood.

If the patient's serum is agglutinating both A1 and A2 cells, this suggests the presence of an unexpected alloantibody reacting in saline at room temperature. See ANTIBODY SCREENING AND ANTIBODY IDENTIFICATION.


1. AABB Technical Manual, 9th Edition, 1985, Pages 287; 660-661.

2. ORTHO Product Literature, Anti-A1 Lectin; Rev. July 1981.


Author:  Peggy Schroeder, revised by Peggy Jensen


Clinical Microbiology Syllabus