I AND P BLOOD GROUP SYSTEMS
I Blood Group System
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
binding between 2 galactose molecules forming a branched oligosaccharides
that make up the I antigen seen in most
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
- 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
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
||Serum + B cells
||Cells + anti-D
||Cells + Rh Cont
||Unknown ABO Group
Method to resolve these Problems
- Correct forward typing by washing cells in warm saline
- 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
- omitting potentiators like LISS and
- using monospecific anti-IgG.
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
Anti-I, Anti-IH, and Anti-H Reactions
||Adult A Cells
||Adult O Cells
||Cord A Cells
||Cord O Cells
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.
- 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.
- High-titer anti-I also commonly associated with
Mycoplasma. pneumoniae infections. Treatment would be giving the
patient appropriate antibiotics.
- Anti-i, which is fairly uncommon and transient, is associated with infectious
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
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
- The Pk antigen occurs when the Pk
antigen does not convert to P
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.
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
- 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
- A negative Donath-Landsteiner = neg at both temps,
OR hemolysis at 4oCas well as 37oC
no hemolysis = NEGATIVE
no hemolysis = NEGATIVE
|4oC + 37oC
incubation: hemolysis: = POSITIVE