6.3 Lewis System

6.3 Lewis System

Quick Answer: Lewis antigens are made by FUT3 (LE) acting with or without FUT2 (Se) and are adsorbed onto red cells from plasma glycolipids; they are not integral membrane proteins. Le(a+b−) = Lewis gene, nonsecretor. Le(a−b+) = Lewis gene, secretor. Le(a−b−) = le/le, more common in African ancestry, and can appear transiently in pregnancy. Lewis antibodies are usually IgM, room-temperature, not clinically significant, do not cause HDFN, and are not routinely honored unless they react at 37 °C / AHG.

Lewis is II.B.2 on the June 9, 2026 outline because candidates keep treating it like Kell. It is not Kell. The antigens are borrowed, the antibodies are usually cold, and the fetal red cell barely carries the antigen. A BB item that hands you anti-Le(a) at immediate spin is asking whether you will waste antigen-negative inventory.

Two fucosyltransferases, three common adult phenotypes

FUT3 (LE), on chromosome 19, is an α-1,4-L-fucosyltransferase. FUT2 (Se) is the secretor α-1,2-L-fucosyltransferase already met in Section 6.1. Both act on type 1 chains in the Golgi of secretory epithelium; the glycolipid products spill into plasma and adsorb onto the red-cell membrane over days. Red cells do not synthesize Lewis antigen with their own FUT3 in any way that matters for phenotyping.

• If the person has LE and is a nonsecretor (sese), FUT3 adds fucose to the type 1 precursor and makes Le^a. Red cells type Le(a+b−). Saliva has Le^a, not ABH.
• If the person has LE and is a secretor (Se), FUT2 usually fucosylates the type 1 chain first (type 1 H). FUT3 then adds a second fucose and makes Le^b. Red cells type Le(a−b+). A little Le^a is made intracellularly, but adult cells typically type Le(a−). Saliva has ABH and Lewis substance.
• If the person is le/le, neither Le^a nor Le^b is made. Red cells type Le(a−b−) regardless of Se. You cannot infer secretor status from a Le(a−b−) type.

Le(a+b+) is uncommon in European adults. It is seen in infants (the phenotype is still converting) and in some Asian partial secretors with a weak Se enzyme that leaves both antigens on the cell.

The interaction is ordered. FUT2 prefers to act first in a secretor. That is why you do not see a stable adult Le(a+b+) as the default secretor type. Teach the three adult boxes, then the infant/partial-secretor exception. Do not invent a fourth everyday phenotype.

Not an intrinsic red-cell antigen

Because Lewis glycolipids are adsorbed, the phenotype is a plasma story wearing a red-cell coat. Transfused Le(a−b−) cells placed in a Le(a−b+) recipient will acquire Le^b from the recipient’s plasma over a few days. Conversely, a patient’s Lewis type can look weaker after massive transfusion of Le(a−b−) cells or after plasma exchange that strips the glycolipid source. You cannot use a post-transfusion Lewis type to genotype the patient, and you cannot assume a donor unit will still type the same after it has lived in the recipient.

Newborns type Le(a−b−) because they have not yet adsorbed enough plasma glycolipid and because FUT3/FUT2 expression is still ramping. They often pass through a Le(a+) or Le(a+b+) stage before reaching the adult type. That developmental sequence is why cord cells are poor Lewis antigen carriers and why Lewis antibodies do not cause HDFN in any practical sense: there is little antigen on the fetal red cell, and the maternal antibody is usually IgM that never crosses the placenta.

Pregnancy lowers plasma Lewis glycolipid. Many pregnant patients phenotype as Le(a−b−) even though they are genetically LE. After delivery the true type returns. Do not permanently relabel a pregnant patient as le/le from one prenatal type, and do not treat a prenatal anti-Le(a) as a new clinically significant alloantibody just because the current cells type Le(a−).

Le(a−b−) is substantially more common in people of African ancestry (often taught near 22%, versus about 6% in European ancestry). That is a genetics fact, not a disease and not an indication that Lewis antibodies will be more dangerous in that population.

Lewis antibodies on the bench

Anti-Le^a is more common than anti-Le^b. Both are usually IgM, react at room temperature or below, may disappear at 37 °C, and are enhanced by enzyme-treated cells (ficin, papain). They can bind complement and occasionally show up in the antiglobulin phase if the tube was never warmed or if complement-binding IgM is still on the cell when AHG is added. Neutralization with saliva or plasma that contains the corresponding Lewis substance is a classic identification trick: incubate the antibody with secretor saliva (or known Lewis-positive plasma), then retest — true Lewis activity is abolished. That neutralization is how you separate Lewis from a cold alloantibody that enzymes also enhance (anti-P1, anti-I, some anti-H).

Clinical significance is the point the exam is hunting:

• Do not cause HDFN. IgM does not cross the placenta, and fetal/newborn red cells carry little Lewis antigen. A group O mother with anti-Le(a) is not an HDFN workup for Lewis.
• Rarely cause hemolytic transfusion reactions. Isolated case reports exist, almost always when the antibody is reactive at 37 °C or in the AHG phase.
• Do not routinely honor Lewis antibodies when selecting red cells. Crossmatch-compatible units at 37 °C / IAT are enough. Phenotype-matched Lewis-negative units are reserved for the uncommon 37 °C–reactive antibody.
• Enzymes enhance Lewis. A screen that “got stronger” after ficin is consistent with Lewis (also Rh, Kidd, I, P1) and argues against MNS/Duffy destruction, but enhancement alone does not make the antibody clinically significant.

If an antibody IDs as anti-Le(a), reacts 3+ at immediate spin, is gone at 37 °C, and the IAT is negative, you report it, you do not phenotype the inventory, and you issue IAT-crossmatch-compatible red cells. That sentence is the whole clinical rule.

Do not treat Lewis like Kell

Kell antigens are integral glycoproteins. Anti-K is almost always IgG, AHG-reactive, and always honored. Lewis antigens are adsorbed glycolipids. Anti-Le^a that reacts only at immediate spin is a nuisance antibody. Issuing K− units for anti-K is mandatory. Issuing Le(a−) units for a room-temperature anti-Le^a is not standard practice and wastes antigen-negative inventory that a patient with anti-K, anti-Fy(a), or anti-Jk(a) may need tonight.

The same trap appears in the other direction: because Lewis can fix complement, a candidate sees in vitro hemolysis at room temperature and assumes intravascular HTR is inevitable. In vitro complement binding at cold temperatures is not the same as a 37 °C, clinically significant hemolysin. Warm the test. Neutralize. Check the IAT. Only then decide whether the antibody graduates from nuisance to honored.

Lewis also changes after transfusion, so “the unit was Le(a+) on the donor tag” does not predict the antigen density a week later in a Le(a−b−) recipient. Kell does not do that. If the stem reads like a Kell stem — honor, phenotype, avoid HDFN — and the antibody is Lewis, the answer that copies the Kell playbook is wrong.

Secretor inference — what Lewis can and cannot tell you

This table is how you answer “which patient is a nonsecretor?” without a saliva test. Only Le(a+b−) gives you the answer for free. Le(a−b+) is a secretor. Le(a−b−) is a shrug — run saliva or molecular FUT2 if the question actually needs secretor status (for example, a Bombay versus para-Bombay workup from Section 6.1).

Exam traps

• Honoring every Lewis antibody the way you honor Kell, Kidd, or Duffy.
• Predicting HDFN from maternal anti-Le^a.
• Calling a pregnant Le(a−b−) patient a lifelong le/le person.
• Inferring secretor status from Le(a−b−).
• Forgetting that enzymes enhance Lewis, so a ficin-destroyed pattern is not Lewis.
• Transfusing based on donor Lewis type as if the antigen were integral and would stay put after infusion.
• Treating room-temperature complement binding as proof of a clinically significant hemolysin.