Infl uenza A viruses (IAVs) cause acute respiratory infections of humans, which are repeated yearly. Human IAV infections are associated with signifi cant morbidity and mortality and therefore they represent a serious health problem. All human IAV strains are originally derived from avian IAVs, which, aft er their adaptation to humans, can spread in the human population and cause pandemics with more or less severe course of the disease. Presently, however, the potential of avian IAV to infect

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... mans and to cause the disease cannot be predicted. Many studies are therefore focused on factors infl uencing the virulence and pathogenicity of IAV viruses in a given host. Th e virus-host interaction starts by virus attachment via the envelope glycoprotein hemagglutinin (HA) to the receptors on the cell surface. In addition to receptor binding, HA mediates also the fusion of viral and endosomal membranes, which follows the virus endocytosis. Th e fusion potential of HA trimer, primed by proteolytic cleavage, is activated by low pH in endosomes, resulting in HA refolding into the fusion-active form. Th e HA conformation change is predetermined by its 3-D structure, is pH-dependent, irreversible and strain-specifi c. Th e process of fusion activation of IAV hemagglutinin is crucial for virus entry into the cell and for the ability of the virus to replicate in the host. Here we discuss the known data about the characteristics of fusion activation of HA in relation to IAV virulence and pathogenicity. ; phone: 421-2-59302427. Abbreviations: HA = hemagglutinin; HA0 = HA precursor; HA1 = heavy chain of HA; HA2 = light chain of HA; HPAI = highly pathogenic avian infl uenza; IAV(s) = infl uenza A virus(es); LPAI = low pathogenic avian infl uenza; M1 protein = matrix protein; mRNA = messenger RNA; NA = neuraminidase; NEP protein = nuclear export protein; NS1 protein = nonstructural protein 1; PA = polymerase acidic protein; PB1 = polymerase basic protein 1; PB2 = polymerase basic protein 2; RBS = receptor binding site; RNA = ribonucleic acid; RNP = ribonucleoprotein; SA = sialic acid; vRNA = viral RNA Content: