Lewis antibodies

The Lewis blood group system (Le^a and Le^b antigens) differs from other blood group systems in that the antigens are not synthesized by RBCs themselves but are produced by secretory tissues (as soluble glycosphingolipids) and then passively adsorbed onto the RBC membrane from plasma. Lewis antigen expression depends on secretor status (FUT2/Se gene) and Lewis gene (FUT3/Le) status: Le(a+b-) individuals are non-secretors, Le(a-b+) are secretors, and Le(a-b-) lack the Lewis gene product entirely. Lewis antigens are also expressed on various epithelial and endothelial cells and can serve as receptors for pathogens (e.g., Helicobacter pylori, norovirus).

Anti-Lewis antibodies (anti-Le^a more clinically relevant than anti-Le^b) arise naturally without prior transfusion or pregnancy sensitization, often in Lewis-negative individuals exposed to Lewis substance in plasma or food. They are usually IgM, react optimally at cold temperatures (4°C), and fix complement but rarely cause significant intravascular hemolysis because Lewis antigens are weakly expressed on RBCs and can be neutralized by soluble Lewis substance in donor plasma before causing damage.

Clinical significance: - Generally considered clinically insignificant for transfusion reactions, though rare cases of acute hemolytic transfusion reactions have been reported, particularly with anti-Le^a reacting at 37°C with complement fixation. - Lewis antibodies are NOT a cause of hemolytic disease of the fetus and newborn (HDFN) because Lewis antigens are poorly developed on fetal RBCs, and the antibodies are predominantly IgM which does not cross the placenta. - During pregnancy, Lewis antigen expression on maternal RBCs often diminishes, leading to transient Le(a-b-) phenotype, making Lewis antibodies commonly detected in antibody screens of pregnant women. - Lewis phenotype has associations with susceptibility to certain infections (H. pylori binds Le^b antigen in gastric mucosa) and altered risk for UTIs (non-secretors more susceptible).

For boards: Know that Lewis antibodies are typically IgM, cold-reacting, clinically insignificant (contrast with Rh, Kidd, Duffy, Kell antibodies which are clinically significant IgG antibodies causing HDFN and delayed hemolytic transfusion reactions). Associate Lewis blood group with secretor status and H. pylori/norovirus receptor biology as a tested concept.