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Posted On : 16 May 2013

what is pi antigen

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Answer 1

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.
Like A and B antigens, they are present on tissue cells as well as red cells.

P Phenotypes

80 percent of people with P antigen are P1 phenotype {have P and P1 antigens} and 20 percent 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 plus 37oC incubation: hemolysis: = POSITIVE

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