The blood brain barrier consisting of astrocytes, pericytes and brain microvascular endothelial cells plays a vital role in the pathogenesis of neurotropic viruses by controlling the access of circulating molecules, immune cells or viruses into the central nervous system (CNS). However, this barrier is not impenetrable and neuroviruses have evolved to disrupt and evade it. This review aims to describe the underlying entry mechanisms of several neuroviruses such as (Japanese encephalitis virus (JEV), West Nile virus (WNV), Zika virus (ZIKV), Nipah virus (NiV), Rabies virus (RABV), Herpes simplex virus (HSV) and Human immunodeficiency virus (HIV)) into the CNS through BBB disruption. The mechanisms, through which neurotropic viruses enter the BBB, are being studied and are becoming clearer, however, some aspects still remain unknown. Some of these viruses are able to invade the brain parenchyma by a 'Trojan horse' mechanism, through diapedesis of infected immune cells that either cross the BBB paracellularly or transcellularly. Important mechanisms of BBB disruption associated with paracellular entry of viruses include alterations in expression or phosphorylation of tight junction proteins, disruption of the basal lamina and disruption of the actin cytoskeleton. In the absence of such mechanisms, indirect effects of viruses on the immune system are likely causes of barrier disruption.

Influenza viruses belong to the family Orthomyxoviridae of enveloped viruses and are an important cause of respiratory infections worldwide. The influenza virus is able to infect a wide variety species as diverse as poultry, marine, pigs, horses, and humans. Upon infection with influenza virus the innate immunity plays a critical role in efficient and rapid control of viral infections as well as in adaptive immunity initiation. The humoral immune system produces antibodies against different influenza antigens, of which the HA-specific antibody is the most important for neutralization of the virus and thus prevention of illness. Cell mediated immunity including CD4+ helper T cells and CD8+ cytotoxic T cells are the other arms of adaptive immunity induced upon influenza virus infection. The complex inherited factors and age related changes are associated with the host immune responses. Here, we review the different components of immune responses against influenza virus. Additionally, the correlation of the immune response to age and inherited factors has been discussed. These determinations lead to a better understanding of the limitations of immune responses for developing improved vaccines to control influenza virus infection.

The findings of this study support the antiviral activity of EBN against influenza virus and validate the traditional usage of this natural remedy by elucidation of toxicity and the molecular mechanism of action.

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