9.1 P1PK, Globoside, and Ii Systems

9.1 P1PK, Globoside, and Ii Systems

Quick Answer: P1 people have P1 + P (and usually undetectable Pk). P2 people lack P1 but still have P. The rare p phenotype lacks P, P1, and Pk and makes anti-PP1Pk (anti-Tja) — a clinically significant hemolysin linked to recurrent spontaneous abortion. Ordinary anti-P1 is a cold IgM and is usually insignificant. I is the branched adult type 2 chain; i is the linear cord chain. Autoanti-I is the cold agglutinin of Mycoplasma CAD; autoanti-i follows EBV. Do not confuse anti-I (all adult cells) with anti-H (O cells strongest).

The June 9, 2026 BB outline splits this material across II.B.5 P1PK / Globoside (P) and II.B.6 Ii. Candidates lose items by treating anti-P1 like anti-K and by ranking anti-I as if it were leftover H.

Two systems, one glycolipid ladder

P1PK (ISBT 003) and GLOB (ISBT 028) share a biosynthetic pathway, not a single gene. The antigens are glycosphingolipids, not integral proteins like Kell or Kidd.

Lactosylceramide is the starting glycolipid. A4GALT (α-1,4-galactosyltransferase) adds galactose and makes Pk (Gb3). B3GALNT1 then adds β-1,3-GalNAc and makes P antigen (globoside, Gb4). Separately, A4GALT can decorate paragloboside and create P1. NOR is a rare A4GALT-variant antigen on this same transferase. Luke (LKE) sits farther down the globoside side: most P+ people are LKE+; p and Pk cells are LKE−. Luke and NOR are name-recognition items — know they live on this pathway, then move on.

P is a high-prevalence antigen. Pk is also high-prevalence but is hard to detect on ordinary P1 or P2 cells because B3GALNT1 converts it onward to P. The p phenotype is an A4GALT null: the ladder never starts, so P1, P, and Pk are all absent. The Pk phenotypes are B3GALNT1 nulls: Pk accumulates and P is missing.

P (globoside) is the cellular receptor for parvovirus B19 — p red cells are resistant. Pk (Gb3) binds Shiga toxin. Those facts appear as “why is this phenotype interesting?” color, not as reasons to change a routine crossmatch.

Anti-P1 is not anti-PP1Pk

Anti-P1 is usually a naturally occurring cold IgM in P2 people. It prefers 4 °C or room temperature, is enhanced by ficin or papain, and can be neutralized by hydatid-cyst fluid, pigeon-egg white, or earthworm extract. Bird handlers and patients with echinococcal (hydatid) disease are the classic associations. It is not clinically significant unless it still reacts at 37 °C. Do not phenotype the refrigerator for a room-temperature-only anti-P1, and do not delay surgery for it.

Anti-PP1Pk (historically anti-Tja) is a different antibody made only by p people. It is often IgM plus IgG, binds complement, hemolyzes in vitro (especially enzyme-treated cells), and is always honored. It causes acute hemolytic transfusion reactions and can cause HDFN. Women with the p phenotype have a high rate of recurrent spontaneous abortion because P and Pk structures are expressed on trophoblast, not because ordinary anti-P1 is dangerous. Compatible red cells are p units — rare-donor registry, siblings, or autologous. Group O cells are not compatible; they carry P and usually P1. Calling the p patient a “universal recipient of O cells” is an ABO habit that will fail this item.

Alloanti-P is made by Pk people and is also significant. Autoanti-P is a different story: it is the Donath-Landsteiner antibody of paroxysmal cold hemoglobinuria (PCH) — a biphasic IgG that binds in the cold and lyses with complement when the tube (or the patient) is warmed to 37 °C. PCH is typically a post-viral illness in children. Do not turn a Donath-Landsteiner stem into a p-phenotype rare-donor problem, and do not turn a p-phenotype panagglutinin into PCH just because the letter P appears.

I is branched; i is linear

I and i are carbohydrate antigens on type 2 chains. They are not antithetical protein alleles like K/k. i is a linear poly-N-acetyllactosamine (repeating Galβ1-4GlcNAc). I is the branched product after GCNT2, the I-branching β-1,6-N-acetylglucosaminyltransferase, acts. Cord cells are i-strong and I-weak. Adult cells are I-strong and i-weak. Branching is largely complete by about 18 months. The rare adult i phenotype lacks branching activity, keeps strong i, and can make alloanti-I — the one setting in which anti-I is an alloantibody rather than a cold auto.

Autoanti-I is the everyday cold autoantibody. Benign cold autoanti-I is low-titer, lives at 4 °C or room temperature, and does not hemolyze the patient. Pathogenic cold agglutinin disease (CAD) is a high-titer, wide-thermal-amplitude autoanti-I that still binds at 30–37 °C, fixes complement, and causes hemolysis. The DAT in CAD is often C3 only. Mycoplasma pneumoniae is the classic infectious association for anti-I CAD. Autoanti-i is the cold auto of infectious mononucleosis (EBV) and some lymphoproliferative disease; it prefers cord cells over adult cells. Reverse those infections on an item and you have missed the pairing the outline wants.

Pathogenicity is thermal amplitude and titer, not the letter I. A titer of 64 at 4 °C that dies at room temperature is a nuisance antibody. A titer of 1024 that still binds at 30 °C is CAD. Do not call every cold auto “clinically significant” and do not ignore a wide-thermal-range auto because “cold antibodies never matter.”

Prewarm versus adsorption

Prewarm (serum and cells brought to 37 °C; no cold phase) can hide a benign cold auto so an underlying alloantibody can be seen. It is the wrong sole method for pathogenic CAD, and it can miss weakly reactive significant alloantibodies — some Kidd, some Vel, and even anti-PP1Pk if you strip the cold phase they use in vitro. Cold autoadsorption (or allogeneic adsorption if the patient was recently transfused) is the safer CAD path. RESt (rabbit erythrocyte stroma) pulls anti-I, anti-IH, anti-H, and anti-P1, and can occasionally pull a significant antibody, so do not treat RESt-adsorbed serum as proven alloantibody-free without thinking.

Recently transfused CAD: do not autoadsorb circulating donor cells and pretend the eluate is the patient’s auto only. Use allogeneic adsorption or adsorption onto enzyme- or ZZAP-treated cells chosen to match the patient’s own phenotype.

Anti-I is not anti-H

This is the highest-yield trap in II.B.5–6.

• Anti-I reacts with all adult cells — group O, A, B, and AB. Cord cells and adult i cells are weak or negative. ABO type does not rank the reactions.
• Anti-H (and often anti-IH) ranks leftover H. Strength is O > A2 > B > A2B > A1 > A1B. Bombay (Oh) cells are H-negative. Cord cells have H; they do not give you the clean negative that cord cells give anti-I.
• Anti-IH in A1 or A1B people wants both I and H, so adult O cells are strongest and cord O cells are weaker than adult O.

If the panel is panreactive in the cold and cord cells are negative, think anti-I. If group O cells are 3+ and A1 cells are weak or negative, think anti-H / anti-IH, not anti-I. If everything including the autocontrol is positive in the cold, think cold auto, then decide benign versus CAD by titer and thermal amplitude.

Exam traps

• Honoring every anti-P1 as if it were anti-PP1Pk.
• Issuing group O red cells to a p patient because “O is universal.”
• Pairing autoanti-I with EBV and autoanti-i with Mycoplasma — that pairing is backwards.
• Using prewarm as the only method in pathogenic CAD.
• Ranking anti-I by ABO type, or ranking anti-H by adult-versus-cord I antigen.
• Forgetting that p women miscarry because placenta carries P/Pk, not because garden-variety anti-P1 is a hemolysin.