Glycosidic exclusion and phenotype formation

ABO (H) phenotype and innate isoagglutinin specificities as they arise from “glycosidic exclusion”. Scheme demonstrating how fucosylation provides ABO(H) “blood group completeness”: The germline-encoded, polyreactive ancestral IgM emerges as a glycan-depleted molecule from non-somatic germ cell maturation and developmental cell renewal or growth processes. Although the specific timing between the encodings of the fucosyl-, GalNAc-, and D-Gal transferases remains an open question, fucosylation occurs on both the cell surface and plasma proteins, followed by the transfer of GalNAc and D-Gal to the cell surfaces and the complementary domains of the differentiating IgM. The glycosidic formation of phenotypes and circulating IgM specificities occurs in identical enzymatic steps. Figure reconstructed from Arend, P. ABO (histo) blood group phenotype development and human reproduction  as they relate to ancestral IgM formation: A hypothesis. Immunobiology 221, 116–127 (2016).