Life's Blood

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I Blood Group System

I/i Antigens

The I antigen is found on almost all adults and is part of the precursor component of the oligosaccharide that forms the A, B, and H antigens. 

There are two types of oligosaccharides: Type 1 and Type 2.  These chains that are part of the ABH antigens form the i antigen found in infants

By the time the child reaches 2 years of age a number of b1-->6 linkages are added between the residual galactose components.  In adults these precursor components are become branched with a b1-->6 binding between 2 galactose molecules forming a branched oligosaccharides that make up the I antigen seen in most adults.

  • Therefore I (almost all adults) are formed from branched sugar chains on precursor substance.
  •  All cord bloods form i antigen  from straight-line sugar chains on precursor substance of the A, B, and H antigens.

  • As indicated:  in most children i is converted to I by the age of 2 years.

Anti-I, Anti-IH, and Anti-i Antibodies

Anti-I antibodies are the most common antibodies found if antibody screenings are done at immediate spin, room temperature.  Their characteristics are:

  • IgM immunoglobulins and therefore are saline agglutinins with their optimal temperature at 4oC.  Since they are IgM they also do not cross the placenta.
  • Anti-I is naturally occurring often due to a Mycoplasma pneumoniae infection or some lymphomas and leukemias. 
  • These antibodies are USUALLY not clinically significant
  • Anti-I reacts with all adult cells (including patient’s own, all reagent cells, all donor cells)
  • Anti-I does not react with cord cells
  • Auto-anti-I is a common “cold agglutinin”

High titers of auto-anti-I can interfere with serological testing:

In ABO typing, the forward grouping may have patient cells coated with IgM autoantibody, so all cells agglutinate.  When this happens all results are positive and the group looks like AB+.  On the reverse grouping anti-I in serum reacts with all adult cells, so A1 and B cells always agglutinate.  All results are positive and the group looks like an O.

Cells + anti-A Cells + anti-B Cells + anti-A,B Serum + A1 cells Serum + B cells Cells + anti-D Cells + Rh Cont Interpretation
4+ 4+ 4+ 4+ 4+ 4+ 4+ Unknown ABO Group

Method to resolve these Problems

  • Correct forward typing by washing cells in warm saline and re-testing.
  • Correct reverse by warming the serum before testing, or adsorbing the cold autoantibody out of the serum.

Crossmatch results will result in all donors being incompatible.  Correct by:

  •  warming the donor cells and patient’s serum before crossmatching,
  • omitting potentiators like LISS  and
  • using monospecific anti-IgG.

Anti-IH antibodies are directed against both I antigens and H antigens.  Therefore they react most strongly with group O adult cells, least with cord cells or A1 cells. 

Comparison of Anti-I, Anti-IH, and Anti-H Reactions

Antibody Adult A Cells Adult O Cells Cord A Cells Cord O Cells
Anti-I 2+ 2+ 0 0
Anti-IH 2+ 4+ 0 2+
Anti-H 0 2+ 0 2+
Anti-i 0 0 2+ 2+

Anti-IH is seen most often in A1 adults and is not clinically significant but may cause problems with the ABO grouping and crossmatch results just as anti-I does.  Follow the same steps for resolving the problem as indicated for anti-I.


  1. Anti-I and anti-IH are only clinically significant when they cause Cold Hemagglutinin Disease (CHD).  In this disease a strong autoanti-I is present.  The blood in patient toes, fingertips, earlobes is cooler than than 37oC and the antibody coats cooled cells, binds complement, and causes hemolysis.  This disease is treated by keeping the patient's extremities warm so the blood is not allowed to cool.
  2. High-titer anti-I also commonly associated with Mycoplasma. pneumoniae infections.  Treatment would be giving the patient appropriate antibiotics.
  3. Anti-i, which is fairly uncommon and transient, is associated with infectious mononucleosis.

P System

Antigens of the P system are Pk, P, P1

Basic structure is the precursor substance, but is called globoside (P) or paragloboside (p) in the P system as the diagram below shows:

from the AABB Technical Manual, 13th, 1999, p.291

  • P antigens poorly developed at birth and therefore do not usually cause Hemolytic Disease of the newborn. 
  • They are present in variable amounts on different red cells and seems to diminish in storage.
  • Like ABO and Ii antigens, they are composed of sugar chains as seen in the accompanying figure.
  • Like A and B antigens, they are present on tissue cells as well as red cells.
  • Similar antigens found in nature (bird droppings, pigeon eggs, hydatid cyst fluid) and these substances can be used to neutralize anti-P in a patient's serum.  These similar antigen solutions can be useful when the antibody is interfering with ABO and crossmatch testing.
  • The Pk antigen occurs when the Pk antigen does not convert to P


P Phenotypes

80% of people with P antigen are P1 phenotype (have P and P1 antigens) and 20% of people with P antigen are P2 (P antigen only - no P1).  The P2 phenotype merely signifies absence of P1 antigen in people with P antigen (globoside).  Individuals who are Pk consistantly have an alloanti-P that is an IgM antibody. The absence of the P antigen is very rare and is designated as p. 

P Antibodies

Alloanti-P1 is the most common of the antibodies in the P system. This antibody is commonly seen in P2 people.  IAlloanti-P1'ss characteristics are:

  • IgM class and therefore is saline reactive, enhanced by cold incubation, may bind complement.
  • Not clinically significant
  • Naturally occurring but not regularly occurring.  It is formed with no known red blood cell stimulus - either pregnancy or transfusion.   If individuals are P1 negative, they do not automatically have anti-P1 i, but it is fairly common
  • Antigen-antibody reactions in P system enhanced by adding albumin to system, or by treating red cells with enzymes

Auto anti-P is an  IgG antibody seen in P1 or P2 persons and causes clinically significant Paroxysmal Cold Hemoglobinuria (PCH).  The Donath-Landsteiner Test helps diagnose PCH.  It is designed to detect biphasic anti-P.  A biphasic antibody reacts with the antigen on the red blood cell at colder temperature, binding complement .  When antigen-antibody-complement complex warms up, the red blood cells are hemolyzed.  Donath-Lansteiner test is performed by incubating patient's serum with his own cells first at 4oC, then at 37oC, and then looking for hemolysis.

  • A positive Donath-Landsteiner = no hemolysis at 4oC, no hemolysis at 37oC, hemolysis only in the tube that was incubated at both temperatures
  • A negative Donath-Landsteiner = neg at both temps, OR hemolysis at 4oCas well as 37oC

Donath-Landsteiner Test

4oC incubation:

no hemolysis = NEGATIVE

37oC incubation:

no hemolysis = NEGATIVE

4oC + 37oC incubation: hemolysis: = POSITIVE



  1. Differentiate between I and i antigens regarding structure and age when developed.
  2. State the optimal temperature of reaction of anti-I.
  3. Explain how anti-I interferes with ABO typing and crossmatch results.
  4. Explain how the above problems may be overcome.
  5. Explain why anti-I is always an autoantibody.
  6. Explain why anti-I or anti IH is not generally clinically significant.
  7. Explain the difference in reactions between anti-I and anti-IH
  8. Describe the cause, symptoms and treatment of Cold Hemagglutinin Disease.
  9. State what other disease auto-anti-I may be associated with.
  10. State what disease anti-i may be associated with.
  11. State the major antigens of the P system, and how they are distributed in the population.
  12. State which antigens make up the following phenotypes in the P system: P1, P2, p
  13. Explain what paragloboside or globoside is.
  14. List three other sources of the P antigen in nature, and explain when this knowledge may become useful.
  15. State the most common allo-antibody in the P system.
  16. Explain how antigen-antibody reactions in the P system may be enhanced.
  17. State the cause of paroxysmal cold hemoglobinuria (PCH).
  18. Describe the Donath-Landsteiner test and explain what it detects.

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Clinical Microbiology Syllabus