The invention provides methods for inhibiting immune responses by inhibiting the biosynthesis of the sialyl galactosides that are involved in immune responses. In particular, B lymphocyte-mediated immune responses are mediated by interfering with synthesis of &agr;2,6 sialylgalactosides, while T lymphocyte-mediated immune responses are inhibited by blocking synthesis of &agr;2,3 sialylgalactosides. The inhibition is accomplished by, for example, inhibiting the activity of a glycosyltransferase involved in synthesis of the respective sialyl galactoside.