Both types A and B produce epidemic disease and type A produces pandemic disease. Since two distinct antigenic lineages of B exist with varying incidence, manufacturers have gained approval to formulate quadrivalent vaccines. Due to the antigenic diversity that is seen in Anemarsaponin-BIII Influenza viruses, new vaccines must be reformulated on an annual schedule. Influenza A and B both contain segmented genomes composed of 8 distinct ribonucleoprotein elements, each containing a negative sense RNA. The surface glycoprotein antigens, hemagglutinin and neuraminidase are both significant immunogens that contribute to the development of an anti-influenza response. In addition to understanding basic virus structure and properties of these viruses, it would also be of value to Procyanidin-B1 vaccine manufactures to determine the quantity and relative amounts of HA and NA on the virion surface of wild type and vaccine candidates. This would aid in determining the best candidate for maximal antigen yield during vaccine production. Furthermore, the quantities and relative amounts of the HA and NA will affect efficacy of the vaccine in producing an immunogenic response. The virus particle shape may also be of importance for high yield production as strains with spherical virions are known to produce higher titer in ovo than strains which are filamentous. Spherical virions may have the biophysical properties necessary for optimal purification of the virus during the gradient centrifugal steps utilized during preparation of the vaccine. Ruigrok et al. found that spherical influenza particles had higher infectivity and a higher ratio of HA to M protein than non-spherical virions. High HA spike quantity is an important property for efficient vaccine production. X-ray crystallography has revealed the atomic structure of HA and the NA head. HA are trimers with an elongated fusion domain, a globular receptor-binding domain and a vestigial esterase domain. NA are club-shaped tetramers with 4stranded anti-parallel b-sheets arranged like the blades of a propeller. Influenza particles are pleomorphic. Laboratory adapted strains are typically spherical with diameters in the range of 100 to 
130 nm, but the virions can also exist as larger ellipsoids or filamentous virions which can be several microns in length. Because influenza is pleomorphic, 3-D structural analysis of the entire virion is not possible by single particle analysis or tomogram averaging. Cryo-EM tomography has been employed to study the surface proteins. Harris et al visualized the 3-D structure of type A H3N2 strain X-31 virus using cryo-EM tomography and determined that a typical 120 nm diameter type A influenza virion can contain up to 375 surface spikes but the actual count could be lower due to viral surface regions with lower spike density. Calder et al. employed cryo-EM tomography to study the structural organization of filamentous influenza A and observed that the interaction between the M1 protein and surrounding envelope determines morphology of the virion. Giocondi et al. used atomic force microscopy to study the 3-D topography of H1N1 influenza and a lateral heterogeneity of the HA and NA spikes was observed for virions at neutral pH and after treatment at pH 5. The distributions of surface glycoproteins on two filamentous type A virus particles has recently been determined. Improvements in the reconstruction algorithms may yield better identification of components of this non-symmetric virus. Marabini et al., demonstrated that Algebraic Reconstruction Techniques using generalized Kaiser-Bessel window functions produced more efficacious 3-D reconstructions in electron microscopy than those produced by alternative methods.