This study describes how site-specific glycosylation of Influenza A virus changes in response to pressures from the host immune system, thereby allowing the virus to evolve and escape neutralization. We compared the glycosylation patterns of different virus strains as they evolve and correlated this information with changes in biological activity. Furthermore, modelling and molecular dynamics studies were performed to understand the basis for glycan microheterogeneity and interactions of the virions with host-immune molecules.