Glycoproteins in mammalian cells are modified with complex-type aspargine-linked glycans of variable chain lengths and composition. Observations of mice carrying mutations in glycosyltransferase genes imply that N-glycan structures regulate T-cell receptor clustering and hence sensitivity to agonists. We argue that the heterogeneity inherent in N-glycosylation contributes to cellular diversity and, thereby, to adaptability in the immune system.