I looked into this a little for you.  I'm not a hematologist or
immunologist, but I can understand some of the tech literature.  If
I've gotten some of this wrong, I hope someone will correct me.

In addition to the ABO and Rh blood groups that everybody is familiar
with, there are some lesser known ones, including the P system.  Just
as with the ABO system, a person naturally has antibodies to whichever
antigens they don't have - e.g. someone with type O blood will have
anti-A and anti-B antibodies.  P1 is a very common trait, so it's less
common, but not rare, to lack it and thus have anti-P1 antibodies.
According the reference below, about 25% of the population have anti-P1
antibodies, i.e. they don't express the P1 antigen.

Where this is important is the case of hemolytic disease of the
newborn.  You may be familiar of the problem of a woman who is Rh- and
carries a Rh+ child.  At birth, there's a little mixing of blood
systems between mother and infant, so the mother may later develop
antibodies to Rh+ positive blood from this exposure, because unlike the
ABO system, these antibodies are only formed with exposure to the Rh+
antigen.  If the woman later bears another Rh+ child, her antibodies
may cross the placenta and attack the fetus's blood cells.  Now that
this is understood, a woman who is Rh- is usually given anti-Rh+
antibodies after the child is born to destroy any of the infant's blood
cells that may have entered her blood before her immune system can
produce anti-Rh+ antibodies of her own.  This protects her future
children from hemolytic disease of the newborn, which is now less
common because of this practice.

Now here's where I'm on shakier ground because I don't remember exactly
how this works!  In the ABO system, and presumably the P system, you
have antibodies to whichever factors you lack even without exposure to
them.  So if the mother and child have incompatibilities, the mixing of
their blood at childbirth can set up a serious immune reaction in the
baby.  The effect on the mother is much less because an adult has so
much more blood than a newborn, and a newborn is such a delicate little
creature.  At one time, and for all I know, still, the treatment for
hemolytic disease of the newborn was to completely replace the baby's
blood with compatible blood, thus removing the antigens, the
antibodies, the breakdown products of the destroyed blood cells, etc.
That's where you and other people with anti-P1 come in -- to provide
compatible blood to save these babies.

The P system is also involved in blood transfusions, but I don't know
if P typing has become standard practice.  Before blood is transfused,
it is cross-matched -- a little blood from the patient and the donation
are mixed, and the result is evaluated to determine if there are any
incompatibilities.  This makes the transfusion much safer even if the P
type is known, because there are other factors, no doubt including some
that have not yet been discovered, so no matter how detailed the
typing, crossmatching is always done.  You may have noticed that after
you donate blood, the technician seals the tube in several places to
make separate little packets of blood.  These samples are used for
testing and crossmatching.

Now as for looking through the literature - this gets complicated.
There are directly relevant papers on problems like spontaneous
abortion (early miscarriage) due to P system incompatibility, problems
caused by incompatibility in transfusion when crossmatching was waived
in dire emergencies, etc.  These papers are embedded in a sea of
others, because the P system antigens are involved in cell surface
adhesion.  Some common bacteria involved in dental caries and urinary
tract infections react with P system markers on the cell surface, so
certain combinations of P factors may make some people more or less
susceptible to these conditions.  It also appears that the P system
antigens are of interest to cancer researchers, presumably to study how
cancer cells lose the inhibition against excess replication normally
exerted by adjacent normal cells, as well as how defects in cell
adhesion enable the cancer cells to metastasize from the original
site.  So if you search on anti-P1, you'll find lots of papers in which
P system antibodies are used as research tools.

I hope this helps, and that more knowledgable people will correct me
where I screwed up.  The best reference I've found online is OMIM -
Online Mendelian Inheritance in Man:

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=OMIM&dopt=
Detailed&tmpl=dispomimTemplate&list_uids=111400

Just skip over the parts you don't understand and see what you can pick
up from the rest.  If the link there doesn't work for you, search OMIM
for "blood group, P system", or #111400.

If you want to correspond with me, delete everything after the 'edu' in
my email address.