P antigen system
The P antigens are carbohydrate antigens that include P1, P, and Pk.
- Pk antigen is a receptor for shiga toxins and Escherichia coli-associated hemolytic uremic syndrome (HUS). It is also a receptor for Streptococcus suis (zoonotic bacterium which can cause bacterial meningitis).
- The P antigen is a receptor for parvovirus B19 (agent causing erythema infectiosum/fifth disease which can cause transient anemia or aplastic crisis).
- P1, P, Pk and LKE antigens all serve as receptors for P-fimbriated uropathogenic E. coli (cause of chronic urinary tract infections).
P phenotypes are define by reactivity to antibodies to anti-P1, anti-P, anti-Pk and anti-PP1Pk.
- P1 Phenotype: anti-P1 (+), anti-P (+) and anti-PP1Pk (+) and anti-Pk (-). Seen in 95% of Blacks and 80% of Whites.
- P2 Phenotype: anti-P1 (-), anti-P (+), antiPP1Pk (+), and anti-Pk (-). Seen in 5% of Blacks and 20% of Whites.
- Rare p phenotype (absence of P antigens): anti-P1 (-), anti-P (-), anti-PP1Pk (-), and anti-Pk (-). These individuals have a very strong anti-PP1Pk which can be associated with delayed hemolytic transfusion reactions and hemolytic disease of the fetus and newborn (HDFN).
- Anti-P1 titers are often elevated in patients with liver flukes (fascioliasis), hydatid cyst disease and who are frequently exposed to birds (which may have P1-like substances in their excrement).
- Anti-PP1Pk is composed of a mixture of anti-P, anti-P1 and antiPk in the serum of p individuals. Alloanti-P is seen in the sera of P1k and P2k persons and is naturally occurring and predominantly IgM isotype (but may also be a mixture of IgM and IgG). The antibodies are capable of causing hemolytic transfusion reactions and hemolytic disease of the fetus and newborn (if IgG isotype and therefore capable of crossing the placenta). There is a relationship between anti-PP1Pk and early spontaneous abortion (the placenta is rich in Pk and P antigens which are targeted by IgG isotype antibodies).
- Autoanti-P (Donath-Landsteiner) is an autoantibody of P specificity and is seen in individuals with paroxysmal cold hemoglobinuria (PCH). In PCH, autoanti-P is of IgG isotype and is a biphasic hemolysin (it is capable of hemolysis only after incubation at two different temperatures in vitro). In the body, autoanti-P antibodies bind to red blood cells at cold temperatures (such as in acral circulation) and, after circulating back to the core of the body, subsequently induce intravascular hemolysis in warmer temperatures (core body temperature).
Anti-P antibodies are not usually detected with routine laboratory methods. It is possible to detect them using the Donath-Landsteiner test. This test is performed on 2 vials of blood at two different temperatures: 4 C and 37 C (body temperature). A test is interpreted as positive only after a patient's red blood cells have been incubated at both temperatures and subsequently hemolyzed.
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- Mais DD. ASCP Quick Compendium of Clinical Pathology, 2nd Ed. Chicago: ASCP Press, 2009.