2-33. P BLOOD GROUP SYSTEM
a. The P blood group system is genetically and chemically complex. The P1
antigen, which is present on the red blood cells of approximately 80 percent of Whites, has
been shown to be closely related to antigens in the ABO system. In fact, the P1 antigen
is chemically very similar to the B antigen. Most red blood cell samples that lack P1
(approximately 20 percent of Whites) are said to be P2. Both P1 and P2 individuals possess
P antigen on their blood cells. Very rare individuals of the phenotype p (previously
known as Tj(a-)) lack all known P antigens from their red cells. Other rare individuals
who lack P from their red blood cells have the so-called Pk antigen present.
b. The phenotypes in this system are shown in Table 2-21.
c. The P1 antigen varies considerably in strength on the cells of different P1-
positive individuals. The antigen deteriorates rapidly on storage and reliable P1 typing
requires the use of fairly fresh red blood cells with sufficiently active antiserum to detect
weak antigens. Cold temperatures enhance this reaction. Anti-P1 is a commonly
encountered, naturally occurring antibody. Some investigators feel it can be detected in
all serums from P2 individuals, if sensitive enough techniques are employed. The
antibody reacts optimally at 4C; relatively fewer examples are detected at 25C, and it
is rare to find examples reacting at 37C (if 37C conditions are strictly controlled). Anti-
P1 rarely causes "in vivo" hemolysis, the unusual examples reacting at 37C being the
main cause for concern. Thus, anti-P1 is of very little clinical importance, causing much
unwarranted anxiety in many transfusion laboratories. As anti-P1 is almost always IgM,
it does not cross the placenta and cause hemolytic disease of the newborn.
Table 2-21. P blood group system.