Oligosaccharides at the surface proteins HA and NA might have greater impact on viral escape from the immune system than single amino acid changes in the antigenic sites. Oligosaccharides which are recognised as "self" by the host immune system may pose conformational changes in the molecule and mask antigenic sites, thereby prevent binding of host antibodies. The number of N-linked glycosylation sites in the H3 HA protein has increased during the years from only three attachment sites in 1968 [58, 59] to ten predicted sites in the Danish isolates after 2004.