In common with a number of parasite surface antigens about which we have heard, the influenza haemagglutinin is a membrane glycoprotein which varies in antigenicity.

A question which often arises concerns the contribution of the carbohydrate side-chains to antigenicity, and we have recently observed that a number of monoclonal antibodies are able to discriminate between glycosylated and nonglycosylated haemagglutinin molecules. Specifically these antibodies select antigenic variants with haemagglutinins which contain asparagine substitutions in the sequence asparagine–cysteine–threonine and the asparagine is glycosylated. By infecting cells incubated in medium containing tunicamycin non-glycosylated variant haemagglutinin can be obtained, and it is possible to show that such molecules are precipitated by the antibodies used to select the variant. These results directly indicate that carbohydrate side-chains can modulate the antigenic properties of glycoproteins and may be involved in antigenic variation.