Cell surface glycans: The why and how of their functionality as biochemical signals in lectin-mediated information transfer

Calculations of the coding capacity of biomolecules come up with the striking result that carbohydrates excel nucleotides and amino acids by far in this respect. Consequently, glycans are ideally suited to equip cell surfaces with a maximum of signals in a minimum of space and form the cellular glycome. The ensuing concept of the sugar code interprets glycans as a platform for encoding messages and shifts in the glycome as a means to swiftly add/remove signals. Next, it implies the presence of effectors that read the signals in situ. Fittingly, recent work on natural decoding devices in mammals (endogenous lectins) has revealed not only their presence but a level of complexity that matches the structural diversity of glycans. The emerging importance of lectin-ligand interaction in immune regulation calls for an introduction to this concept for nonspecialists. This primer to the fundamentals of the sugar code, with telling examples from the realm of immunology, also illustrates the strategic and hitherto unsuspected fine-tuning between glycan structure and lectin design/expression.