Antiviral Immunity

Viruses are obligate intracellular parasites that cause infection by invading cells of the body. Their life cycle comprises a relatively short extracellular period, and a longer intracellular period during which they undergo replication. The immune system has non-specific and specific mechanisms that attack the virus in both phases of its life cycle. Nonspecific immune response is mainly mediated by interferons and natural killer cells (NK). Type I interferons are produced by many cell types and lead to both inhibition of viral replication and cell proliferation; they also enhance the ability of NK to lyse infected cells. NK represent a different lineage of lymphocytes that recognize and lyse virally infected cells. They are mainly effective during an early stage of viral infection, since there is no lag phase of clonal expansion for NK as occurs with T and B lymphocytes. Specific immune antiviral mechanisms are both humoral and cellular. Specific antibodies protect against viral infections and play an important role in antiviral immunity, mainly during the early stage of the infection. The most effective antiviral antibodies are neutralizing antibodies which bind to the viral envelope or capsid proteins, and block the virus from entering into host cell. The main effectors involved in specific antiviral immunity are CD8+ cytotoxic T lymphocytes (CTL). These cells recognize viral antigens presented at the cell surface associated with class I MHC molecules. CTL response is not always beneficial, since the tissue destruction caused by CTL is sometimes greater than the damage done by the virus.